Prism optical element, image observation apparatus and image display apparatus

ABSTRACT

An extremely compact prism optical element, image observation apparatus and image display apparatus which are capable of providing an observation image that is clear and has minimal aberration and minimal distortion even at a wide field angle. Light rays emitted from an image display device ( 7 ) enter an ocular optical system ( 12 ) through a fourth surface ( 6 ) and are totally reflected toward an observer&#39;s pupil ( 1 ) by a third surface ( 5 ). The reflected light rays are reflected by a first surface ( 3 ) disposed immediately in front of the observer&#39;s pupil ( 1 ) and then reflected toward the observer&#39;s pupil ( 1 ) by a second surface ( 4 ). The reflected light rays pass through the first surface ( 3 ) and are projected into an observer&#39;s eyeball ( 15 ) with the observer&#39;s iris position as an exit pupil ( 1 ). When an external-scene image is observed, light rays from an object point in,the external scene enter the ocular optical system ( 12 ) through the third surface ( 5 ), pass through the first surface ( 3 ) and are projected into the observer&#39;s eyeball ( 15 ). Assuming that the angle of internal reflection of an arbitrary light ray at the third surface ( 5 ) is θ r3 , the ocular optical system ( 12 ) satisfies the condition of sin −1 (1/n d )≦θ r3 ≦60°, where n d  is the refractive index for the spectral d-line of the medium of the ocular optical system ( 12 ).

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a prism optical element, animage observation apparatus and an image display apparatus. Moreparticularly, the present invention relates to a head- or face-mountedimage display apparatus that can be retained on the observer's head orface.

[0002] As an example of conventional head- or face-mounted image displayapparatus, an image display apparatus disclosed in Japanese PatentApplication Unexamined Publication (KOKAI) No. 3-101709 (1991) is known.In this image display apparatus, an image that is displayed by an imagedisplay device is transmitted as an aerial image by a relay opticalsystem including a positive lens, and the aerial image is projected intoan observer's eyeball as an enlarged image by an ocular optical systemformed from a concave reflecting mirror.

[0003] U.S. Pat. No. 4,669,810 discloses another type of conventionimage display apparatus. In this apparatus, an image of a CRT istransmitted through a relay optical system to form an intermediateimage, and the image is projected into an observer's eye by acombination of a reflection holographic element and a combiner having ahologram surface.

[0004] U.S. Pat. No. 4,026,641 discloses another type of conventionalimage display apparatus. In the conventional image display apparatus, animage of an image display device is transferred to a curved objectsurface by an image transfer device, and the image transferred to theobject surface is projected in the air by a toric reflecting surface.

[0005] U.S. Reissued Pat. No. 27,356 discloses another type ofconventional image display apparatus. This apparatus is an ocularoptical system designed to project an object surface onto an exit pupilby a semi-transparent concave mirror and a semitransparent plane mirror.

[0006] Other known image display apparatuses include those which aredisclosed in U.S. Pat. Nos. 4,322,135 and 4,969,724, European Patent No.0,583,116A2, and Japanese Patent Application Unexamined Publication(KOKAI) No. 7-333551 (1995).

[0007] However, an image display apparatus of the type in which an imageof an image display device is relayed, as in Japanese Patent ApplicationUnexamined Publication (KOKAI) No. 3-101709 (1991) and U.S. Pat. No.4,669,810, must use several lenses as a relay optical system in additionto an ocular optical system, regardless of the type of ocular opticalsystem. Consequently, the optical path length increases, and the opticalsystem increases in both size and weight.

[0008] Because a head-mounted image display apparatus is fitted to thehuman body, particularly the head, if the amount to which the apparatusprojects from the user's face is large, the distance from the supportingpoint on the head to the center of gravity of the apparatus is long.Consequently, the weight of the apparatus is imbalanced when theapparatus is fitted to the observer's head. Further, when the observermoves or turns with the apparatus fitted to his/her head, the apparatusmay collide with something. That is, it is important for a head-mountedimage display apparatus to be small in size and light in weight. Anessential factor in determining the size and weight of the apparatus isthe arrangement of the optical system.

[0009] However, if an ordinary magnifier alone is used as an ocularoptical system, exceedingly large aberrations are produced, and there isno device for correcting them. Even if spherical aberration can becorrected to a certain extent by forming the configuration of theconcave surface of the magnifier into an aspherical surface, otheraberrations such as coma and field curvature remain. Therefore, if thefield angle for observation is increased, the image display apparatusbecomes impractical. Alternatively, if a concave mirror alone is used asan ocular optical system, it is necessary to use not only ordinaryoptical elements (lens and mirror) but also a device for correctingfield curvature by an image transfer device (fiber plate) having asurface which is curved in conformity to the field curvature produced.

[0010] In an image display apparatus of the type in which an image of animage display device is projected into an observer's eyeball by using atoric reflecting surface as in U.S. Pat. No. 4,026,641, field curvaturethat is produced by the decentered toric reflecting surface is correctedby curving the object surface itself. Therefore, it is difficult to usea flat display, e.g. an LCD (Liquid Crystal Display), as an imagedisplay device.

[0011] In a coaxial ocular optical system in which an object surface isprojected on an observer's pupil by using a semitransparent concavemirror and a semitransparent plane mirror as in U.S. Reissued Pat. No.27,356, because two semitransparent surfaces are used, the brightness ofthe image is reduced to as low a level as {fraction (1/16)}, even in thecase of a theoretical value. Further, because field curvature that isproduced by the semitransparent concave mirror is corrected by curvingthe object surface itself, it is difficult to use a flat display, e.g.an LCD (Liquid Crystal Display), as an image display device.

SUMMARY OF THE INVENTION

[0012] In view of the above-described problems of the conventionaltechniques, an object of the present invention is to provide anextremely compact image observation apparatus and image displayapparatus which are capable of providing an observation image that isclear and has minimal aberration and minimal distortion even at a widefield angle, and a prism optical element for use in these apparatuses.

[0013] To attain the above-described object, the present inventionprovides a prism optical element formed from a plurality of surfacesfacing each other across a medium having a refractive index (n) largerthan 1 (n>1). The prism optical element has a first surface, a secondsurface, a third surface, and a fourth surface. The first surface hasboth a transmitting action through which light rays enter the prismoptical element or exit therefrom and a reflecting action by which lightrays are internally reflected in the prism optical element. The secondsurface is disposed to face the first surface across the medium and hasa reflecting action by which light rays are internally reflected in theprism optical element. The third surface is disposed substantially closeto the second surface to face the first surface across the medium andhas a reflecting action by which light rays are internally reflected inthe prism optical element. The fourth surface has such a transmittingaction that when the first surface has an action through which lightrays enter the prism optical element, the fourth surface has an actionthrough which light rays exit from the prism optical element, whereas,when the first surface has an action through which light rays exit fromthe prism optical element, the fourth surface has an action throughwhich light rays enter the prism optical element. The prism opticalelement satisfies the following condition:

sin⁻¹(1/n _(d))≦θ_(r3)≦60°  (1)

[0014] where n_(d) is the refractive index for the spectral d-line ofthe medium, and θ_(r3) is the angle of internal reflection of anarbitrary light ray at the third surface.

[0015] In the present invention, the arrangement of the second and thirdsurfaces is not necessarily limited to the one in which surfacesdesigned separately from each other are disposed adjacent to each other,but includes an arrangement in which the second and third surfaces areformed by using one identical surface such that one region of thesurface acts as the second surface, and another region of the surfaceacts as the third surface. In this case, an overlap region that acts asboth the second and third surfaces may be present because a bundle oflight rays has a width.

[0016] One image observation apparatus according to the presentinvention has an image forming device and an ocular optical systemhaving an action by which an image formed by the image forming device isled to an eyeball of an observer. The ocular optical system includes aprism member having at least three surfaces. The space between the atleast three surfaces is filled with a single medium having a refractiveindex (n) larger than 1 (n>1). The prism member has an action by whichlight rays emitted from the image forming device are internallyreflected at least three times. At least two of the at least threeinternal reflections are total reflections. At least one of the at leasttwo total reflections is performed by a surface disposed on a side ofthe single medium that is closer to the observer. The surface is curvedso as to correct aberrations produced by the internal reflections in theprism member. At least two of the at least three surfaces of the prismmember are disposed to face each other such that an external scene canbe observed through the at least two surfaces, and that a distortionproduced when the external scene is observed through the single mediumis minimized.

[0017] Another image observation apparatus according to thepresent-invention has an image forming device and an ocular opticalsystem having an action by which an image formed by the image formingdevice is led to an eyeball of an observer. The ocular optical systemincludes at least a prism member. The prism member has at least fouroptical surfaces having a transmitting or reflecting optical action. Thespace surrounded by the at least four optical surfaces is filled with asingle medium having a refractive index (n) larger than 1 (n>1). The atleast four optical surfaces include a first surface, a second surface, athird surface, and a fourth surface. The first surface has both atransmitting action and a reflecting action and is disposed on a side ofthe prism member that is closer to the observer's eyeball. The secondsurface has a reflecting action and is disposed to face the firstsurface across the medium. The second surface is at least decentered ortilted with respect to the observer's visual axis. The third surface hasa reflecting action and is disposed to face the first surface across themedium at a position substantially adjacent to the second surface. Thefourth surface is disposed such that one end thereof is substantiallyadjacent to the first surface, and the other end thereof issubstantially close to the third surface. At least the third surface hasa totally reflecting action. The first surface, the single medium andthe third surface are arranged to have an external scene observationaction by which an external scene can be observed through the firstsurface, the single medium and the third surface.

[0018] Still another image observation apparatus according to thepresent invention has an image forming device and an ocular opticalsystem having an action by which an image formed by the image formingdevice is led to an eyeball of an observer. The ocular optical systemincludes at least a prism member. The prism member has at least fouroptical surfaces having a transmitting or reflecting optical action. Thespace surrounded by the at least four surfaces is filled with a singlemedium having a refractive index (n) larger than 1 (n>1). The at leastfour optical surfaces include a first surface, a second surface, a thirdsurface, and a fourth surface. The first surface has both a transmittingaction and a reflecting action and is disposed on a side of the prismmember that is closer to the observer's eyeball. The second surface hasa reflecting action and is disposed to face the first surface across themedium. The second surface is at least decentered or tilted with respectto the observer's visual axis. The third surface has a reflecting actionand is disposed to face the first surface across the medium at aposition substantially adjacent to the second surface. The fourthsurface is disposed such that one end thereof is substantially adjacentto the first surface, and the other end thereof is substantially closeto the third surface. At least the second or third surface has a totallyreflecting action. In addition, a line-of-sight detecting device fordetecting an observer's line of sight is disposed near a totallyreflecting region of the second or third surface that has a totallyreflecting action.

[0019] An image display apparatus according to the present invention hasan image display device and an ocular optical system for leading animage formed by the image display device to an eyeball of an observersuch that the image can be observed as a virtual image. The ocularoptical system includes a decentered prism in which a space formed by atleast two surfaces is filled with a medium having a refractive indexlarger than 1. The at least two surfaces include a first surfacepositioned immediately in front of the observer's eyeball, and a secondsurface which is a reflecting surface facing the first surface. At leastone of the at least two surfaces is a curved surface decentered ortilted with respect to the observer's visual axis. The ocular opticalsystem further includes an aberration correcting device disposed outsidethe second surface to correct aberrations due to decentration producedby the first and second surfaces with respect to light from an externalscene.

[0020] Another image display apparatus according to the presentinvention has an image display device and an ocular optical system forleading an image formed by the image display device to an eyeball of anobserver such that the image can be observed as a virtual image. Theocular optical system includes a decentered prism in which a spaceformed by at least three surfaces is filled with a medium having arefractive index larger than 1. The at least three surfaces include arefracting and internally reflecting surface positioned immediately infront of the observer's eyeball; an outside world-side internallyreflecting surface disposed on the outside world side of the ocularoptical system to face the refracting and internally reflecting surface;and a refracting surface through which a bundle of light rays emittedfrom the image display device enters the decentered prism. At least oneof the at least three surfaces is decentered or tilted with respect tothe observer's visual axis. The at least three surfaces are arranged toperform at least three internal reflections. The ocular optical systemfurther includes a second optical element that cancels a power producedby the refracting and internally reflecting surface, which is positionedimmediately in front of the observer's eyeball, and the outsideworld-side internally reflecting surface with respect to external lightwhen an external scene is observed through the two surfaces. The secondoptical element is disposed on the outside world side of the outsideworld-side internally reflecting surface.

[0021] In the present invention, the arrangement of the second and thirdsurfaces is not necessarily limited to-the one in which surfacesdesigned separately from each other are disposed adjacent to each other,but includes an arrangement in which the second and third surfaces areformed by using one identical surface such that one region of thesurface acts as the second surface, and another region of the surfaceacts as the third surface. In this case, an overlap region that acts asboth the second and third surfaces may be present because a bundle oflight rays has a width.

[0022] The arrangements and operations of the prism optical element,image observation apparatus and image display apparatus according to thepresent invention will be described. In the description of the imageobservation apparatus and image display apparatus in particular, theexplanation will be made on the basis of backward ray tracing in whichlight rays are traced from the observer's pupil position toward theimage display device for the convenience of designing the opticalsystem, unless otherwise specified.

[0023] In the image observation apparatus according to the presentinvention, light rays from the image display device (image formingdevice) are internally reflected three times in the ocular opticalsystem, thereby enabling the optical path to be folded very effectively,and thus realizing an extremely thin ocular optical system. Two of thethree internal reflections are specified as total reflections.Consequently, the area that requires reflection coating is markedlyreduced, thereby succeeding in realizing a compact, lightweight andlow-cost ocular optical system. By specifying two of the threereflections as total reflections, it is possible to minimize theincidence of a ghost image due to the occurrence of unwanted light orthe reduction in contrast-caused by flare. Usually, in an optical systemhaving internal reflection and filled with an optical medium having arefractive index larger than 1, the influence of unwanted light emergingfrom an image display device at a large exit angle and unwanted lightdue to reflection in a path other than the proper ray path gives rise toa problem. In the present invention, the number of reflection coatingsurfaces is reduced by using two totally reflecting surfaces.Consequently, unwanted light other than the desired bundle of light raysemanating from the image display device and reaching the observertspupil is transmitted by the two internally reflecting surfaces. Thus,unwanted light reaching the observer's pupil is markedly reduced.

[0024] The above-described action will be described in detail withreference to FIGS. 20(a) and 20(b). FIGS. 20(a) and 20(b) arefragmentary enlarged views showing a part of a decentered prism 12through which light from an image display device 7 enters. Thedecentered prism 12 has three surfaces 101, 102 and 103 decentered withrespect to the optical axis, and the space formed by the surfaces 101,102 and 103 is filled with a medium having a refractive index largerthan 1. Reference numeral 15 denotes an observer's eyeball. Referencenumeral 101 denotes an entrance surface. Reference numeral 102 denotesan outside world-side reflecting or totally reflecting surface.Reference numeral 103 denotes an observer-side refracting or reflectingsurface. FIG. 20(a) shows an arrangement in which the reflecting surface102 is provided with reflection coating. FIG. 20(b) shows an arrangementin which the reflecting surface 102 is a totally reflecting surface,which is provided-with no reflection coating. In the case of FIG. 20(a),light emitted from the left-hand end of the image display device 7 at alarge exit angle enters the decentered prism 12 while being refracted bythe entrance surface 101. The incident light is reflected by thereflecting surface 102, which is provided with reflection coating. Thereflected light passes through the refracting surface 103 and enters theobserver's eyeball 15. Accordingly, the observer sees an unwantedelectronic image in the upper part of the observer's visual field inaddition to the proper image (hereinafter referred to as “electronicimage”) of the image display device 7. Alternatively, flare appears inthe upper part of the observer's visual field.

[0025] In the case of FIG. 20(b), light emitted from the left-hand endof the image display device 7 at a large exit angle enters thedecentered prism 12 while-being refracted by the entrance surface 101,and is incident on the reflecting surface 102, which is provided with noreflection coating. Because the incident angle is smaller than thecritical angle, the unwanted light passes through the reflecting surface102. Accordingly, the unwanted light is transmitted to a side of thedecentered prism 12 that is remote from the observer and therefore doesnot enter the observer's eyeball 15. In other words, neither a ghostimage nor flare occurs.

[0026] The above-described action can be similarly caused to take placeat any totally reflecting surface in addition to the reflecting surfacein this example. That is, the above-described action can be attained bysetting the optical system such that a bundle of light rays in the raypath for observation of the proper electronic image has an incidentangle not smaller than the critical angle, and that any other ray bundlehaving an exit angle which may cause ghost or flare has an incidentangle smaller than the critical angle at a totally reflecting surface.It becomes easy to obtain the above-described effect by setting twototally reflecting surfaces as described above. Thus, it becomespossible to provide the observer with a clear observation image which isfree from a ghost image and which has a minimal reduction in contrastdue to flare.

[0027] First, assuming that the prism optical element according to thepresent invention is used as an ocular optical system (observationoptical system) of an image observation apparatus or an image displayapparatus, the prism optical element is formed from a prism member inwhich at least three internal reflections take place, and the prismmember is filled with a medium having a refractive index larger than 1.Therefore, the ocular optical system can be made extremely thin by theabove-described optical path folding effect. In addition, aberrationcorrection can be made even more effectively by the arrangement in whichat least three internal reflections take place. Thus, it is possible topresent an observation image which is clear as far as the edges of theimage field. In this regard, the prism optical element according to thepresent invention will be described below more specifically.

[0028] The principal power of the prism optical element is given by thesecond surface, which is a reflecting surface. In this case, the secondsurface can be formed with a large radius -of curvature in comparison toa refracting system of the same power as that of the reflecting surface.Therefore, aberrations produced by the second surface can be minimized.Further, the outside world-side reflecting surface is divided into twodifferent surfaces (i.e. the second and third surfaces). Therefore, itbecomes possible to set the direction of reflected light as desiredindependently of the curvature of each surface. Accordingly, the opticalsystem can be so shaped as to conform to the curve of the observer'sface, and the image display device can be disposed such that the back ofthe device faces the observer. In particular, when the image displaydevice is an LCD or the like that requires a back light, the back lightand the associated electric system are provided on the observer side.Therefore, no part of the image display device projects forwardly, andthe amount to which the whole image display apparatus projects from theobserver's face can be minimized.

[0029] In general, when a concave mirror is decentered or tilted withrespect to the optical axis, aberrations due to decentration areproduced, which do not occur in a coaxial system. In the case of theprism optical element according to the present invention also, when itis used in an image observation apparatus or an image display apparatus,aberrations due to decentration are produced because the second surfaceis decentered or tilted with respect to the observer's visual axis. Inparticular, astigmatism and coma occur even on the optical axis becausethere is a difference in power between a direction along a planecontaining the optical path of the axial principal ray (i.e. tangentialdirection) and a direction perpendicular to a plane containing thevisual axis and the optical path of the axial principal ray (i.e.sagittal direction). The aberrations due to decentration produced by thesecond surface can be corrected by forming at least one of the at leastfour surfaces constituting the prism optical element from a surfacehaving different powers in the tangential and sagittal directions, i.e.a rotationally asymmetric surface.

[0030] An even more effective way of correcting the aberrations due todecentration is to use a surface-having only one plane of symmetry toform at least one of the surfaces constituting the prism opticalelement. When the image display device is disposed on the observer'svisual axis (i.e. axial principal ray), a bilaterally symmetricobservation image can be projected into the observer's eyeball by usinga surface having a plane of symmetry in the sagittal direction as atleast one of the surfaces constituting the ocular optical system. On theother hand, if the surface is arranged to have no plane of symmetry inthe tangential direction, the degree of freedom in the tangentialdirection increases, and it is possible to even more favorably correctdecentration aberrations occurring in a plane containing the opticalpath of the axial principal ray.

[0031] When the above-described ocular optical system comprises at leastfour surfaces, reflection taking place at the first surface may be totalreflection. If the first surface, which is a surface disposedimmediately in front of the observerts pupil, is a totally reflectingsurface, a region through which light rays exit from the ocular opticalsystem and an internally reflecting region can be arranged to overlapeach other. In other words, a single surface can be arranged to performboth transmitting and reflecting actions. Accordingly, it is possible toconstruct a compact ocular optical system.

[0032] In addition, it is possible to provide a clearer observationimage because the above-described ghost and flare reducing effect by thetotally reflecting surface can also be obtained at the first surface.Further, because only the second surface requires reflection coating,the productivity improves, and a lower-cost image display apparatus canbe realized.

[0033] In the above-described prism optical element, it is desirable tosatisfy the following condition:

sin⁻¹(1/n _(d))≦θ_(r3)≦60°  (1)

[0034] where n_(d) is the refractive index for the spectral d-line ofthe medium, and θ_(r3) is the angle of internal reflection of anarbitrary light ray at the third surface.

[0035] It is important to satisfy the condition (1). By setting θ_(r3)equal to or greater than sin⁻¹ (1/n_(d)), the angle of internalreflection at the third surface becomes equal to or greater than thecritical angle. Consequently, it is possible for an arbitrary light rayemitted from the image display device to be totally reflected at thethird surface.

[0036] If the angle of reflection at the third surface is excessivelylarge, the prism optical element becomes undesirably long in thedirection (tangential direction) perpendicular to the visual axis. Inthe case of a wide-field image display apparatus in particular,extra-axial rays diverge to such an extent that the rays cannot reachthe first surface, which is the subsequent reflecting surface.Consequently, it becomes impossible to realize the desired image displayapparatus. Accordingly, it is desirable that at the third surface theangle of internal reflection of an arbitrary light ray emitted from theimage display device should be set not larger than the upper limit ofthe condition (1), i.e. 60°.

[0037] It is more desirable to satisfy the following condition:

sin⁻¹(1/n _(d))≦θ_(r3)≦50°  (2)

[0038] Because the third surface is a curved surface tilted ordecentered with respect to the optical axis (axial principal ray), theangle of reflection at this surface should be as small as possible. Thesmaller the reflection angle, the smaller the amount of aberrationcaused by decentration, particularly comatic aberration due todecentration. Accordingly, it is desirable that at the third surface theangle of internal reflection of an arbitrary light ray emitted from theimage display device should be set not greater than the upper limit ofthe condition (2), i.e. 50°.

[0039] It is important from the viewpoint of realizing a low-cost imagedisplay apparatus to use a plane surface as at least one of the surfacesconstituting the prism optical element. By doing so, another surface canbe defined on the basis of the at least one plane surface. Therefore, itis possible to facilitate the mechanical design and production of anoptical system. Consequently, it is possible to shorten the machining orprocessing time and to facilitate the layout of the whole apparatus.Thus, it is possible to realize substantial reductions in manufacturingcosts.

[0040] Similar advantageous effects can be obtained by using a sphericalsurface as at least one of the surfaces constituting the prism opticalelement. In this case, another surface can be readily defined on thebasis of the at least one spherical surface. Therefore, the layout ofthe whole apparatus is also facilitated. Thus, it is possible to realizesubstantial reductions in manufacturing costs.

[0041] It is desirable that the refractive index n of the mediumconstituting the prism optical element should be larger than 1.3.

[0042] It will be clear from the foregoing description that anobservation optical apparatus can be constructed by disposing theabove-described prism optical element in an observation optical system.

[0043] In this case, the prism optical element may be disposed in anobjective lens. Alternatively, the prism-optical element may be disposedin an image erecting device which is disposed behind an objective lensto erect an object image formed by the objective lens. In the latterarrangement, the prism optical element can be arranged to have both animage erecting action and an ocular lens action.

[0044] The prism optical element according to the present invention canbe used to construct a head-mounted image display apparatus having animage forming device consisting essentially of an LCD (Liquid CrystalDisplay) or a CRT disposed to face the fourth surface of the prismoptical element, or an image-forming device consisting essentially of anLCD, a CRT or the like which is relayed by a relay optical system. Thehead-mounted image display apparatus further has a retaining memberadapted to retain both the prism optical element and the image formingdevice on the observer's face. A bundle of light rays emitted from theimage forming device enters the prism optical element through the fourthsurface and passes sequentially along the optical path in the prismoptical element. More specifically, the incident ray bundle is reflectedsuccessively by the third surface, the first surface and the secondsurface and exits from the prism optical element through the firstsurface.

[0045] In the present invention, the second and third surfaces may beformed from a single identical surface. In this case, the number ofphysical surfaces can be reduced by one, and-it is therefore possible tosimplify the process in terms of the optical design and the productionof prism and hence possible to contribute to the achievement of anincrease in mass-productivity and reductions in costs. It is desirablethat a physically single surface should be arranged to have both thefunctions of the second and third surfaces, and that internallyreflecting regions should overlap each other. By doing so, it ispossible to realize a reduction in the size of the prism member.

[0046] One image observation apparatus according to the presentinvention has an image forming device and an ocular optical systemhaving an action by which an image formed by the image forming device isled to an eyeball of an observer. The ocular optical system includes aprism member having at least three surfaces. The space between the atleast three surfaces is filled with a single medium having a refractiveindex (n) larger than 1 (n>1). The prism member has an action by whichlight rays emitted from the image forming device are internallyreflected at least three times. At least two of the at least threeinternal reflections are total reflections. At least one of the at leasttwo total reflections is performed by a surface disposed on a side ofthe single medium that is closer to the observer. The surface is curvedso as to correct aberrations produced by the internal reflections in theprism member. At least two of the at least three surfaces of the prismmember are disposed to face each other such that an external scene canbe observed through the at least two surfaces, and that a distortionproduced when the external scene is observed through the single mediumis minimized.

[0047] In the image observation apparatus according to the presentinvention, the third surface 5 is a totally reflecting surface, which isprovided with no reflection coating. Therefore, external light passingthrough the third surface 5 and the first surface 3 reaches theobserver's eyeball 15. Accordingly, it is possible to observe anexternal scene in a range α different from an electronic imageobservation range β. The fact that the observer can observe an externalscene image and an electronic image in different partial regions of thevisual field means that the observer can simultaneously observe theexternal scene in the upper region of the observer's visual field andthe electronic image in the lower region of the visual field, by way ofexample. It should, however, be noted that the partial regions of thevisual field for observation of different images may be divided in anydirection and in any form, e.g. upper and lower regions, or left andright regions, as long as the observer can observe the different imagesin the respective partial regions. This function enables the observer torecognize the outside world with the image observation apparatus mountedon his or her head or face. Thus, it is possible to provide a safe imageobservation apparatus that enables the observer to avoid a dangeroussituation and to cope with an emergency situation. Consequently, therange of applications of the image observation apparatus widens.

[0048] In this image observation apparatus, it is desirable that theimage forming device should be an image display device, e.g. an LCD or aCRT disposed such that the image forming surface thereof faces thefourth surface (it should be noted that an image display device which isrelayed by a relay optical system is not expected as the image formingdevice in the image observation apparatus), and that the second surfaceshould be formed from a curved surface.

[0049] The above-described image observation apparatus can beconstructed as a head-mounted image display apparatus by providing aretaining member that retains both the image display device and theocular optical system in front of an observer's eyeball, and arrangingthe prism member such that a bundle of light rays emitted from the imagedisplay device enters the prism member through the fourth surface, andthe incident ray bundle is reflected successively by the third surface,the first surface and the second surface so as to exit from the firstsurface.

[0050] In the above-described image observation apparatus, the prismmember may be fixed at the same position regardless of whether theobserver views the image formed by the image forming device or an imageof an external scene. In this case, it is desirable that the image fromthe image forming device and the external-scene image should be capableof being observed in the respective partial regions through the firstand third surfaces, as described later with reference to FIG. 7.

[0051] The prism member may be provided with a switching device thatmoves the prism member so as to enable observation modes to changebetween the observation of the image formed by the image forming deviceand the observation of the external-scene image.

[0052] More specifically, if the prism member is moved such that thefirst surface of the ocular optical system, which is disposedimmediately in front of the observer's eyeball, and the third surface,which is disposed on the outside world side of the ocular optical systemto totally reflect a part of the principal rays, lie in the vicinity ofthe observer's visual axis, the observer can view an external-sceneimage around-the observer's visual axis lying when he or she looksstraight ahead, i.e. in the vicinity of the center of the visual field.Therefore, the observer can confirm the external scene in front ofhis/her eye with the image display apparatus mounted on his/her head orface. Accordingly, it is possible to realize an image display apparatusensured safety.

[0053] If the electronic image is kept displayed, the external scene canbe confirmed by moving and returning the ocular optical system tothereby switch the external-scene image and the electronic image fromeach other. Accordingly, the range of applications widens.

[0054] In this case, it is desirable for the switching device to movethe prism member such that an optical path extending from the prismmember to the observer's eyeball to observe the image formed by theimage forming device is approximately coincident with an optical pathextending from the prism member to the observer's eyeball to observe theexternal-scene image.

[0055] If the prism member is adapted to move along a plane containingthe optical path of the axial principal ray, the movement of the prismmember is rectilinear. Therefore, the arrangement of the movingmechanism is simplified, and the layout of the whole apparatus isfacilitated. Consequently, a low-cost image display apparatus can berealized.

[0056] If the prism member is movable in a direction perpendicular tothe visual axis, the layout of the whole apparatus is facilitated, andthe arrangement of the moving mechanism is simplified. Moreover, becausethere is no change in the amount to which the ocular optical systemprojects forward from the observer's face even after the movement of theocular optical system, a small-sized and compact image display apparatuscan be provided.

[0057] If the prism member is rotatable, it is possible to observe theexternal scene by moving the prism member through a simple rotatingmechanism. Therefore, the mechanism itself becomes less costly.Moreover, if the prism members for the left and right eyes aresimultaneously rotated, the external scene can be confirmed with botheyes. Accordingly, the safety is enhanced, and the layout of theapparatus can be simplified.

[0058] Another image observation apparatus according to the presentinvention has an image forming device and an ocular optical systemhaving an action by which an image formed by the image forming device isled to an eyeball of an observer. The ocular optical system includes atleast a prism member. The prism member has at least four opticalsurfaces having a transmitting or reflecting optical action. The spacesurrounded by the at least four optical surfaces is filled with a singlemedium having a refractive index (n) larger than 1 (n>1). The at leastfour optical surfaces include a first surface, a second surface, a thirdsurface, and a fourth surface. The first surface has both a transmittingaction and a reflecting action and is disposed on a side of the prismmember that is closer to the observer's eyeball. The second surface hasa reflecting action and is disposed to face the first surface across themedium. The second surface is at least decentered or tilted with respectto the observer's visual axis. The third surface has a reflecting actionand is disposed to face the first surface across the medium at aposition substantially adjacent to the second surface. The fourthsurface is disposed such that one end thereof is substantially adjacentto the first surface, and the other end thereof is substantially closeto the third surface. At least the third surface has a totallyreflecting action. The first surface, the single medium and the thirdsurface are arranged to have an external scene observation action bywhich an external scene can be observed through the first surface, thesingle medium and the third surface. It should be noted that the term asurface other than the four optical surfaces” as used in thisdescription means a prism side surface or a cut surface which has nooptical action.

[0059] These image observation apparatuses are arranged such that anexternal scene can be observed through the surface disposed immediatelyin front of the observer's eyeball and the surface disposed on theoutside world side of the ocular optical system. The action and effectof this arrangement will be described below with reference to FIG. 7.FIG. 7 is a sectional view of a decentered prism 12 in which a spaceformed by four surfaces 3, 4, 5 and 6 which are decentered with respectto an optical axis is filled with a medium having a refractive indexlarger than 1. In the figure, reference numeral 1 denotes an observer'spupil; 2 denotes an observer's visual axis; 3 denotes a first surface ofthe decentered prism 12; 4 denotes a second surface of the decenteredprism 12; 5 denotes a third surface of the decentered prism 12; 6denotes a fourth surface of the decentered prism 12; 7 denotes an imagedisplay device; 15 denotes an observer's eyeball; and 16 denotes anoptical filter. In the actual path of light rays from the image displaydevice 7, light rays emitted from the image display device 7 enter thedecentered prism 12 through the fourth surface 6 and are totallyreflected by the third surface 5. The reflected light rays are totallyreflected by the first surface 3 and then reflected by the secondsurface 4. Then, the reflected light rays pass through the first surface3 to project the image of the image display device 7 into the observer'seyeball 15 with the observer's pupil 1 as an exit pupil.

[0060] In the image observation apparatuses according to the presentinvention, the third surface 5 is a totally reflecting surface, which isprovided with no reflection coating. Therefore, external light passingthrough the third surface 5 and the first surface 3 reaches theobserver's eyeball 15. Accordingly, an external scene can be observed ina range α different from an electronic image observation range β. Thefact that the observer can observe an external scene image and anelectronic image in different partial regions of the visual field meansthat the observer can simultaneously observe the external scene in theupper region of the observer's visual field and the electronic image inthe lower region of the visual field, by way of example. It should,however, be noted that the partial regions of the visual field forobservation of different images may be divided in any direction and inany form, e.g. upper and lower-regions, or left and right regions, aslong as the observer can observe the different images in the respectivepartial regions. This function enables the observer to recognize theexternal scene with the image observation apparatus mounted on his orher head or face. Thus, it is possible to provide a safe imageobservation apparatus that enables the observer to avoid a dangeroussituation and to cope with an emergency situation. Consequently, therange of applications of the image observation apparatus widens.

[0061] In this image observation apparatus, it is desirable that theimage forming device should be an image display device, e.g. an LCD or aCRT disposed such-that the image forming surface thereof faces thefourth surface (it should be noted that an image display device which isrelayed by a relay optical system is not expected as the image formingdevice in the image observation apparatus)-, and that the second surfaceshould be formed from a curved surface.

[0062] The above-described image observation apparatus can beconstructed as a head-mounted image display apparatus by providing aretaining member that retains both the image display device and theocular optical system in front of an observer's eyeball, and arrangingthe prism member such that a bundle of light rays emitted from the imagedisplay device enters the prism member through the fourth surface, andthe incident ray bundle is reflected successively by the third surface,the first surface and the second surface so as to exit from the firstsurface.

[0063] In the above-described image observation apparatus, it isdesirable to arrange the surface disposed immediately in front of theobserver's eyeball and the surface disposed on the outside world side ofthe ocular optical system such that the composite power of the twosurfaces for external light at respective arbitrary positions thereof isapproximately zero. If the composite power for external light isapproximately zero, conditions under which an image of an external sceneis observed become approximately equal to those for observation with thenaked eye, and the external scene can be observed even more naturally.Accordingly, when a dangerous or emergency situation occurs, theexternal scene can be accurately recognized. Consequently, a remarkablysafe image display apparatus can be provided.

[0064] In this case, the first and third surfaces may be formed fromcurved, spherical or plane surfaces. When the observer views theexternal scene, light rays from the external scene pass through thetotally reflecting region of the internally reflecting surface disposedon the outside world side of the ocular optical system and furtherthrough the refracting surface disposed immediately in front of theobserver's eyeball and are projected into the observer's pupil. If thetwo surfaces are not aspherical surfaces but spherical surfaces, an evenmore natural external-scene image can be readily observed at an off-axisposition because there is no change in the curvature of each surface. Ifthe first surface, which is disposed immediately in front of theobserver's eyeball, and the third surface, which is disposed on theoutside world side of the ocular optical system, are plane surfaces, anatural external-scene image can be observed because each surface has nopower. In a case where the two surfaces are perpendicular to theobserver's visual axis and parallel to each other, the external scene isobserved through merely transparent plates. Accordingly, it is possibleto observe an extremely natural external-scene image.

[0065] Assuming that φ_(t1) denotes the composite power for externallight at respective arbitrary regions of the surface disposedimmediately in front of the observer's eyeball and the surface disposedon the outside world side of the ocular optical system, it is desirableto satisfy the following condition:

−0.5≦φ_(t1)≦0.5 (1/millimeter)   (3)

[0066] where φ_(t1) corresponds to each of the power φ_(t1) (yz) in aplane containing the axial principal ray and the power φ_(t1) (xz) in aplane perpendicular to the plane containing the axial principal ray.

[0067] By satisfying the condition (3), the magnification for externallight passing through the decentered prism can be set in theneighborhood of 1. Therefore, it is possible to observe an even morenatural external-scene image.

[0068] In the above-described image observation apparatus, the prismmember may be fixed at the same position regardless of whether theobserver views an image formed by the image forming device or anexternal-scene image. In this case, it is desirable that the image fromthe image forming device and the external-scene image should be capableof being observed in the respective partial regions through the firstand third surfaces, as stated above with reference to FIG. 7.

[0069] The prism member may be provided with a switching device thatmoves the prism member so as to enable observation modes to changebetween the observation of the image formed by the image forming deviceand the observation of the external-scene image.

[0070] More specifically, if the prism member is moved such that thefirst surface of the ocular optical system, which is disposedimmediately in front of the observer's eyeball, and the third surface,which is disposed on the outside world side of the ocular optical systemto totally reflect a part of the principal rays, lie in the vicinity ofthe observer's visual axis, the observer can view an external-sceneimage around the observer's visual axis lying when he or she looksstraight ahead, i.e. in the vicinity of the center of the visual field.Therefore, the observer can confirm the external scene in front ofhis/her eye with the image display apparatus mounted on his/her head orface. Accordingly, it is possible to realize an image display apparatusensured safety.

[0071] If the electronic image is kept displayed, the external scene canbe confirmed by moving and returning the ocular optical system tothereby switch the external-scene image and the electronic image fromeach other. Accordingly, the range of applications widens.

[0072] In this case, it is desirable to arrange the surface disposedimmediately in front of the observer's eyeball and the surface disposedon the outside world side of the ocular optical system such that thecomposite power of the two surfaces for external light at respectivearbitrary positions thereof is approximately zero. If the compositepower for external light is approximately zero, the external scene canbe observed even more naturally. Accordingly, it is possible to avoid adangerous situation and to cope appropriately with an emergencysituation. Consequently, a remarkably safe image display apparatus canbe provided.

[0073] Assuming that φ_(t2) denotes the composite power for externallight at respective arbitrary positions of the surface disposedimmediately in front of the observer's eyeball and the surface disposedon the outside world side of the ocular optical system, it is desirableto satisfy the following condition:

−0.5≦φ_(t2)≦0.5 (1/millimeter)   (4)

[0074] where φ_(t2) corresponds to each of the power φ_(t2) (yz) in aplane containing the axial principal ray and the power φ_(t2) (xz) in aplane perpendicular to the plane containing the axial principal ray.

[0075] By satisfying the condition (4), the magnification for externallight passing through the decentered prism can be set in theneighborhood of 1. Therefore, it is possible to observe an even morenatural external-scene image.

[0076] In this case, it is desirable for the switching device to movethe prism member such that an optical path formed from the prism memberto the observer's eyeball to observe an image formed by the imageforming device is approximately coincident with an optical path formedfrom the prism member to the observer's eyeball to observe anexternal-scene image.

[0077] If the prism member is adapted to move along a plane containingthe optical path of the axial principal ray, the movement of the prismmember is rectilinear. Therefore, the arrangement of the movingmechanism is simplified, and the layout of the whole apparatus isfacilitated. Consequently, a low-cost image display apparatus can berealized.

[0078] If the prism member is movable in a direction perpendicular tothe visual axis, the layout of the whole apparatus is facilitated, andthe arrangement of the moving mechanism is simplified. Moreover, becausethere is no change in the amount to which the ocular optical systemprojects forward from the observer's face even after the movement of theocular optical system, a small-sized and compact image display apparatuscan be provided.

[0079] If the prism member is rotatable, it is possible to observe theexternal scene by moving the prism member through a simple rotatingmechanism. Therefore, the mechanism itself becomes less costly.Moreover, if the prism members for the left and right eyes aresimultaneously rotated, the external scene can be confirmed with botheyes. Accordingly, the safety is enhanced, and the layout of theapparatus can be simplified.

[0080] Still another image observation apparatus according to thepresent invention has an image forming device and an ocular opticalsystem having an action by which an image formed by the image formingdevice is led to an eyeball of an observer. The ocular optical systemincludes at least a prism member. The prism member has at least fouroptical surfaces having a transmitting or reflecting optical action. Thespace surrounded by the at least four surfaces is filled with a singlemedium having a refractive index (n) larger than 1 (n>1). The at leastfour optical surfaces include a first surface, a second surface, a thirdsurface, and a fourth surface. The first surface has both a transmittingaction and a reflecting action and is disposed on a side of the prismmember that is closer to the observer's eyeball. The second surface hasa reflecting action and is disposed to face the first surface across themedium. The second surface is at least decentered or tilted with respectto the observer's visual axis. The third surface has a reflectingaction-and is disposed to face the first surface across the medium at aposition substantially adjacent to the second surface. The fourthsurface is disposed such that one end thereof is substantially adjacentto the first surface, and the other end thereof is substantially closeto the third surface. At least second or third surface has a totallyreflecting action. In addition, a line-of-sight detecting device fordetecting an observer's line of sight is disposed near a totallyreflecting region of the second or third surface that has a totallyreflecting action. In this case also, the term “a surface other than thefour optical surfaces” means a prism side surface or a cut surface whichhas no optical action.

[0081] The action and effect of the image observation apparatus whenconstructed as an image display apparatus will be described below.Disposing the line-of-sight detecting device near the optical systemmakes it possible to detect the observer's line of sight. Detection ofthe observer's line of sight will be described below with reference toFIGS. 5(a), 5(b) and 6. FIG. 5(a) is a sectional view of an imagedisplay apparatus comprising an image display device 7 and a decenteredprism 12 in which a space formed by three surfaces 3, 4 and 6 decenteredwith respect to an optical axis is filled with a medium having arefractive index larger than 1. FIG. 5(b) is a sectional view of animage display apparatus comprising an image display device 7 and adecentered prism 12 in which a space formed by four surfaces 3, 4, 5 and6 decentered with respect to an optical axis is filled with a mediumhaving a refractive index larger than 1. FIG. 6 is a sectional view ofanother image display apparatus comprising an image display device 7 anda decentered prism 12 in which a space formed by four surfaces 3, 4, 5and 6 decentered with respect to an optical axis is filled with a mediumhaving a refractive index larger than 1. In these figures, referencenumeral 1 denotes an observer's pupil; 2 denotes an observer's visualaxis; 3 denotes a first surface of an ocular optical system 12; 4denotes a second surface of the ocular optical system 12; 5 denotes athird surface of the ocular optical system 12; 6 denotes a fourthsurface of the ocular optical system 12; 7 denotes an image displaydevice; 9 denotes a line-of-sight detecting optical system; 10 denotes aline-of-sight detector; 11 denotes an illuminating device; 12 denotes anocular optical system; and 15 denotes an observer's eyeball.

[0082] In the arrangement shown in FIG. 5(a), a line-of-sight detectingdevice comprising the line-of-sight detecting optical system 9 and theline-of-sight detector 10 is disposed on the outside world side of adecentered prism constituting the ocular optical system 12 so as to facethe observer's eyeball 15 across the decentered prism. In this case, theimage of the observer's pupil 1 needs to pass through the first surface3, which is disposed immediately in front of the observer's pupil 1, andthe second surface 4, which is a reflecting surface disposed on theoutside world side of the ocular optical system 12, to enter theline-of-sight detecting device (9 and 10). However, the second surface4, which is disposed on the outside world side of the ocular opticalsystem 12, is a reflecting surface and hence provided with reflectioncoating. Accordingly, it is necessary in order to lead the image of theobserver's pupil 1 to the line-of-sight detecting device (9 and 10) toprovide the reflecting surface with a non-coated portion NC (i.e. a holein the reflection coating). Such a non-coated portion NC would have anadverse effect on the image to be observed.

[0083] FIGS. 5(b) and 6 show image display apparatuses according to thepresent invention. The third surface 5, which is a reflecting surfacedisposed on the outside world side of the ocular optical system 12, isprovided such that a part of the third surface 5 totally reflects lightincident thereon. The totally reflecting portion of the third surface 5can reflect light from the image display device 7 without the need ofreflection coating. Accordingly, it is unnecessary to provide reflectioncoating. Thus, the image of the observer's pupil 1 passes through thefirst surface 3, which is disposed immediately in front of theobserver's pupil 1, and further through the totally reflecting portionof the third surface 5, which is disposed on the outside world side ofthe ocular optical system, and it can be detected by the line-of-sightdetecting device (9 and 10). Therefore, the observer's line of sight canbe detected without the need for providing the reflecting surface of theocular optical system 12 with a coating hole, which has an adverseeffect on the observation of the electronic image.

[0084] In this case, it is desirable for the first surface of thedecentered prism to have a totally reflecting action. In such a case, itis desirable that the line-of-sight detecting device should be disposedat a position where it detects the observer's line of sight through thetotally reflecting region of the second or third surface.

[0085] It is desirable for the image display apparatus to have anilluminating device that illuminates the observer's eyeball. Byilluminating the observer's eyeball, the image display apparatus candetect a bright image, and it is therefore possible to accurately detectthe observer's line of sight. It is desirable that the illuminatingdevice should be provided on the outside world side of the ocularoptical system. If an illuminating device 11 is disposed between theobserver's face and the ocular optical system 12 as shown in FIG. 5(a),the illuminating device 11 is likely to interfere with the observer'sglasses or other thing. However, if the illuminating device 11 isdisposed on the outside world side of the ocular optical system 12, itis possible to avoid an interference between the illuminating device 11and the observer's face. If the illuminating device 11 is provided suchthat illuminating light from the illuminating device 11 passes throughthe totally reflecting portion of the reflecting surface of the ocularoptical system 12, the observer's pupil can be illuminated withoutproviding a coating hole.

[0086] It is desirable to use an illuminating device employing infraredlight. Observation of the electronic image means that the observer'spupil is illuminated by light from the image display device. In the caseof a line-of-sight detecting device that needs an image analysis bycapturing a feeble virtual image, e.g. cornea reflection method, it isnecessary to eliminate a reflection image formed by a bundle of lightrays from the image display device, the illumination light quantity ofwhich varies every moment. Usually, the image display device is an LCDor the like, which emits light in the visible wavelength region.Accordingly, the use of infrared light as light emitted from theilluminating device makes it possible to reduce the influence of lightfrom the image display device.

[0087] In this case also, the image observation apparatus can beconstructed as a head-mounted image display apparatus by providing aretaining member that retains the ocular optical system, the imageforming device and the line-of-sight detecting device in front of anobserver's face.

[0088] The above-described image observation apparatus may have a devicefor positioning the image forming device and the ocular optical systemwith respect to the observer's head.

[0089] Further, it is possible to observe a stereoscopic image with botheyes by providing a device for supporting at least a pair of such imageobservation apparatuses at a predetermined spacing.

[0090] Next, an image display apparatus according to the presentinvention has an image display device and an ocular optical system forleading an image formed by the image display device to an eyeball of anobserver such that the image can be observed as a virtual image. Theocular optical system includes a decentered prism in which a spaceformed by at least two surfaces is filled with a medium having arefractive index larger than 1. The at least two surfaces include afirst surface positioned immediately in front of the observer's eyeball,and a second surface which is a reflecting surface facing the firstsurface. At least one of the at least two surfaces is a curved surfacedecentered or tilted with respect to the observer's visual axis. Theocular optical system further includes an aberration correcting devicedisposed outside the second surface to correct aberrations due todecentration produced by the first and second surfaces with respect tolight from an external scene.

[0091] In this image display apparatus, when an image of an externalscene is observed through the first surface, which is positionedimmediately in front of the observer's eyeball, and the second surface,which is a reflecting surface disposed to face the first surface, theexternal-scene image is observed in the same way as in the case ofobserving it through a lens having power asymmetric with respect to theoptical axis because at least one of the first and second surfaces isdecentered or tilted with respect to the observer's visual axis.Therefore, if an aberration correcting device, e.g. a Fresnel lens,which is arranged to cancel the eccentric power, is disposed on theoutside world side of the second surface, it becomes possible for theobserver to view an even more natural external-scene image. Further,because a Fresnel lens is an extremely thin optical element, it ispossible to provide a compact image display apparatus without causing anincrease in the size of the apparatus.

[0092] According to the present invention, the Fresnel lens may bereplaced by another optical element, e.g. a diffractive optical elementor a holographic optical element, provided that the above-describedeffect can be obtained.

[0093] When a Fresnel lens is used, it is desirable that the center ofthe annular zone of the Fresnel lens should lie in a plane containingthe optical path of the axial principal ray from the image displaydevice, and that the Fresnel lens should be decentered perpendicularlyto the visual axis in the plane containing the optical path of the axialprincipal ray. If the Fresnel lens has an axially symmetricconfiguration, the apparatus becomes excellent in productivity, and theproduction cost can be reduced. If a Fresnel lens having an axiallysymmetric power is disposed in the plane containing the optical path ofthe axial principal ray in such a manner as to be decentered withrespect to the visual axis, aberrations due to decentration which areproduced by the first and second surfaces with respect to external lightcan be corrected even more favorably.

[0094] The Fresnel lens may be disposed such that the center of theannular zone of the Fresnel lens should lie in the plane containing theoptical path of the axial principal ray, and that the Fresnel lensshould be tilted with respect to the visual axial so as to extend alongthe surface configuration of the second surface. When disposed to extendalong the surface configuration of the second surface, the Fresnel lensis tilted with respect to the observer's visual axis. Accordingly, it-ispossible to set a power asymmetric with respect to the optical axis andhence possible to correct even more favorably aberrations due todecentration which are produced by the first and second surfaces withrespect to external light. Moreover, the amount to which the apparatusprojects from the observer's face reduces, and the space between theocular optical system and the Fresnel lens also reduces. Accordingly, itis possible to provide a remarkably compact image display apparatushaving no useless space.

[0095] Another image display apparatus according to the presentinvention has an image display device and an ocular optical system forleading an image formed by the image display device to an eyeball of anobserver such that the image can be observed as a virtual image. Theocular optical system includes a decentered prism in which a spaceformed by at least three surfaces is filled with a medium having arefractive index larger than 1. The at least three surfaces include arefracting and internally reflecting surface positioned immediately infront of the observer's eyeball; an outside world-side internallyreflecting surface disposed on the outside world side of the ocularoptical system to face the refracting and internally reflecting surface;and a refracting surface through which a bundle of light rays emittedfrom the image display device enters the decentered prism. The at leastthree surfaces are arranged to perform at least three internalreflections. The ocular optical system further includes a second opticalelement that cancels the power produced by the refracting andinternally-reflecting surface, which is positioned immediately in frontof the observer's eyeball, and the outside world-side internallyreflecting surface with respect to-external light when an external sceneis observed through the two surfaces. The second optical element isdisposed on the outside world side of the outside world-side internallyreflecting surface.

[0096] When an image of an external scene is observed through the firstsurface, which is positioned immediately in front of the observer'seyeball, and the second surface, which is a reflecting surface disposedto face the first surface, the external-scene image is observed in thesame way as in the case of observing it through a lens having aneccentric power different for each image height because at least one ofthe first and second surfaces is decentered or tilted with respect tothe observer's visual axis. Therefore, if the second optical element,which is adapted to cancel the eccentric power produced by the twosurfaces with respect to external light, is disposed on the outsideworld side of the ocular optical system, it becomes possible for theobserver to view an even more natural external-scene image in a widerange. Accordingly, it is possible to provide a safe image displayapparatus which enables the observer to avoid a dangerous situation andto cope with an emergency situation.

[0097] In this case, the ocular optical system may be formed from adecentered prism in which a space formed by four surfaces is filled witha medium having a refractive index larger than 1. The four surfacesinclude a first surface positioned on the observer's eyeball side andserving as both refracting and reflecting surfaces; a second surfacewhich is a reflecting surface disposed to face the first surface; athird surface which is a reflecting surface disposed to face the firstsurface at a position adjacent to the second surface; and a fourthsurface which is a refracting surface closest to the image displaydevice. At least one of the four surfaces is decentered or tilted withrespect to the observer's visual axis. In a case where the ocularoptical system comprises four surfaces as described above, the externalscene is recognized by-external light passing through the first andsecond surfaces. In this case, it is possible to realize an addedfunction without increasing the overall size of the ocular opticalsystem by disposing the second optical element adapted to cancel theeccentric power only at a region covering the second surface.

[0098] It is desirable to dispose at least one second optical element onthe outside world side of the second or third surface so that theexternal scene can be observed through the first surface, the secondsurface and the second optical element or through the first surface, thethird surface and the second optical element. If the second opticalelement having the action of canceling the eccentric power is providedon the outside world side of the second surface, a naturalexternal-scene image can be observed in approximately the same region asthe region for observation of the electronic image. If the secondoptical having the action of canceling the eccentric power is providedon the outside world side of the third surface, a natural external-sceneimage can be observed in a region different from the region forobservation of the-electronic image. If two second optical elements aredisposed on the outside world sides of the second and third surfaces,respectively, the observer can view all the external-scene image passingthrough the first and second surfaces and through the first and thirdsurfaces. Accordingly, the field angle for observation of theexternal-scene image becomes wider than the field angle for observationof the electronic image. Thus, a natural and wide external-scene imagecan be observed. Consequently, it is possible to provide a remarkablysafe image display apparatus which enables the observer to avoid adangerous situation and to cope appropriately with an emergencysituation.

[0099] It is desirable for the second optical element to be capable ofsimultaneously canceling the composite power of the first and secondsurfaces and the composite power of the first and third surfaces withrespect to external light. If the second optical element capable ofsimultaneously canceling the composite powers is formed from a singleoptical element and it is disposed on the outside world side of theocular optical system, it is possible to observe the external scene overa wide range. Because the second optical element simultaneously cancelsthe composite powers, there is no break in the external-scene image.Thus, it is possible to observe an even more natural external-scene isimage. Accordingly, the external scene can be recognized over a widerange with a single optical element, and it is possible to provide animage display apparatus of reduced cost and enhanced safety whichenables the observer to avoid a dangerous situation and to cope with anemergency situation.

[0100] The above-described image observation apparatus may have a devicefor positioning the image display device and the ocular optical systemwith respect to the observer's head. It becomes possible for theobserver to see a stable electronic image by providing a device forpositioning both the image display device and the ocular optical systemwith respect to the observer's head.

[0101] The image display apparatus may have a device for supporting boththe image display device and the ocular optical system with respect tothe observer's head such that the apparatus can be mounted on theobserver's head. By allowing both the image display device and theocular optical system to be mounted on the observer's head with asupporting device, it becomes possible for the observer to see theobservation image in a desired posture and from a desired direction.

[0102] Further, it is possible to provide a device for supporting atleast a pair of image display apparatuses at a predetermined spacing. Itbecomes possible for the observer to see the electronic image with botheyes without fatigue by providing a device for supporting at least twoimage display apparatuses at a predetermined spacing. Further, if imageswith a disparity therebetween are displayed on the right and left imagedisplay devices, and these images are observed with both eyes, it ispossible to enjoy viewing a stereoscopic image.

[0103] The ocular optical system in the above-described image displayapparatus can be used as an image-forming optical system. If the ocularoptical system is arranged to form an image of an object at infinitywith the image display surface in the ocular optical system-defined asan image plane, the ocular optical system can be used as animage-forming optical system, e.g. a finder optical system for a camerasuch as that shown in FIG. 24.

[0104] It should be noted that in the present invention, the second andthird surfaces may be formed from a single identical surface. In thiscase, the number of physical surfaces can be reduced by one, and it istherefore possible to simplify the process in terms of the opticaldesign and the production of prism and hence possible to contribute tothe achievement of an increase in mass-productivity and reductions-incosts. It is desirable that a physically single surface should bearranged to have both the functions of the second and third surfaces,and that internally reflecting regions should overlap each other. Bydoing so, it is possible to realize a reduction in the size of the prismmember.

[0105] Still other objects and advantages of the invention will in partbe obvious and will in part be apparent from the specification.

[0106] The invention accordingly comprises the features of construction,combinations of elements, and arrangement of parts which will beexemplified in the construction hereinafter set forth, and the scope ofthe invention will be indicated in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0107]FIG. 1 is a sectional view of an optical system for a single eyeof a head-mounted image display apparatus which uses an ocular opticalsystem according to Example 1 of the present invention.

[0108]FIG. 2 is a sectional view of an optical system for a single eyeof a head-mounted image display apparatus which uses an ocular opticalsystem according to Example 2 of the resent invention.

[0109]FIG. 3 is a sectional view of an optical system for a single eyeof a head-mounted-image display apparatus which uses an ocular opticalsystem according to Example 3 of the present invention.

[0110]FIG. 4 is a sectional view of an optical system for a single eyeof a head-mounted image display apparatus which uses an ocular opticalsystem according to Example 4 of the present invention.

[0111] FIGS. 5(a) and 5(b) are sectional views for describing an opticalsystem for a single eye of a head-mounted image display apparatus whichuses an ocular optical system according to Example 5 of the presentinvention in comparison to a modification, in which FIG. 5(a) shows themodification, and FIG. 5(b) shows Example 5.

[0112]FIG. 6 is a sectional view of an optical system for a single eyeof a head-mounted image display apparatus which uses an ocular opticalsystem according to Example 6 of the present invention.

[0113]FIG. 7 is a sectional view of an optical system for a single eyeof a head-mounted image display apparatus which uses an ocular opticalsystem according to Example 7 of the present invention.

[0114] FIGS. 8(a), 8(b) and 8(c) are sectional views of an opticalsystem for a single eye of a head-mounted image display apparatus whichuses an ocular optical system according to Example 8 of the presentinvention.

[0115] FIGS. 9(a), 9(b) and 9(c) are sectional views of an opticalsystem for a single eye of a head-mounted image display apparatus whichuses an ocular optical system according to Example 9 of the presentinvention.

[0116]FIG. 10 is a sectional view of an optical system for a single eyeof a head-mounted image display apparatus which uses an ocular opticalsystem according to Example 10 of the present invention.

[0117]FIG. 11 is a sectional view of an optical system for a single eyeof a head-mounted image display apparatus which uses an ocular opticalsystem according to Example 11 of the present invention.

[0118]FIG. 12 is a sectional view of an optical system for a single eyeof a head-mounted image display apparatus which uses an ocular opticalsystem according to Example 12 of the present invention.

[0119]FIG. 13 is a sectional view of an optical system for a single eyeof a head-mounted image display apparatus which uses an ocular opticalsystem according to Example 13 of the present invention.

[0120]FIG. 14 is a sectional view of an optical system for a single eyeof a head-mounted image display apparatus which uses an ocular opticalsystem according to Example 14 of the present invention.

[0121]FIG. 15 is a sectional view of an optical system for a single eyeof a head-mounted image display apparatus which uses an ocular opticalsystem according to Example 15 of the present invention.

[0122]FIG. 16 is a sectional view of an optical system for a single eyeof a head-mounted image display apparatus which uses an ocular opticalsystem according to Example 16 of the present invention.

[0123]FIG. 17 is a sectional view of an optical system for a single eyeof a head-mounted image display apparatus which uses an ocular opticalsystem according to Example 17 of the present invention.

[0124]FIG. 18 is a sectional view of an ocular optical system arrangedas in Example 17, which is provided with a line-of-sight detectingdevice.

[0125] FIGS. 19(a), 19(b) and 19(c) show a mechanism for moving anocular optical system arranged as in Example 17 to change observationpositions from an electronic image observation position to anexternal-scene image observation position, and also show a direction ofmovement for changing the observation positions.

[0126] FIGS. 20(a) and 20(b) are fragmentary sectional views fordescribing an action in which unwanted light is eliminated by a totallyreflecting surface in the present invention.

[0127]FIG. 21 shows an image display apparatus according to the presentinvention which is arranged in the form of an image display apparatusfor a single eye.

[0128]FIG. 22 shows an image display apparatus according to the presentinvention which is arranged in the form of an image display apparatusfor both eyes.

[0129]FIG. 23 shows an arrangement of an optical system according to thepresent invention which is used as an image-forming optical system.

[0130]FIG. 24 shows an arrangement of an optical system according to thepresent invention which is used as an image-forming optical system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0131] Examples 1 to 17 of the image display apparatus according to thepresent invention will be described below.

[0132] In constituent parameters of each example (described later), asshown typically in FIG. 1, an exit pupil 1 of an ocular optical system12 is defined as the origin of the optical system, and an optical axis 2is defined by a light ray passing through both the center of the displayarea of an image display device 7 and the center (the origin) of theexit pupil 1. A Z-axis is taken in a direction in which light raystravel from the exit pupil 1 along the optical axis 2. A Y-axis is takenin a direction extending through the center of the exit pupil 1 at rightangles to the Z-axis in a plane in which light rays are bent by theocular optical system 12. An X-axis is taken in a direction extendingthrough the center of the exit pupil 1 at right angles to both the Z-and Y-axes. A direction in which the Z-axis extends from the exit pupil1 toward the ocular optical system 12 is defined as a positive directionof the Z-axis. A direction in which the Y-axis extends from the opticalaxis 2 toward the image display device 7 is defined as a positivedirection of the Y-axis. A direction in which the X-axis constitutes aright-handed system in combination with the Z- and Y-axes is defined asa positive direction of the X-axis. It should be noted that ray tracingis carried out by backward tracing from the exit pupil 1 of the ocularoptical system 12, which is defined as the object side, toward the imagedisplay device 7, which is defined as the image plane side.

[0133] Regarding each surface for which displacements Y and Z and tiltangle θ are shown, the displacement Y is a distance by which the surfaceis displaced in the Y-axis direction from the exit pupil 1, which is theorigin of the optical system, while the displacement Z is a distance bywhich the surface is displaced in the Z-axis direction from the exitpupil 1, and the tilt angle θ is an angle of inclination of the centeraxis of the surface with respect to the Z-axis unless otherwisespecified in the constituent data (specified in Examples 6 and 9). Itshould be noted that, for the tilt angle, the counterclockwise directionis defined as a positive direction. In a case where a reference surfaceis particularly specified, displacements and tilt angle are similarlygiven with respect to the vertex of the reference surface.

[0134] In the constituent parameters (shown later), the surfaceseparation in the coaxial portion is shown as the distance from thesurface concerned to the next surface. In addition, the radius ofcurvature of each spherical surface, refractive index of each medium,and Abbe's number are given according to the conventional method.

[0135] FIGS. 1 to 4 and 5(b) to 17 are sectional views of image displayapparatuses according to Examples 1 to 4 and 5 to 17 of the presentinvention, taken along a plane containing the optical axis in theexamples shown in FIGS. 1 to 4, 5(b) to 11, 15 and 16, the image displayapparatuses each comprise a decentered prism 12 in which a space formedby four surfaces 3, 4, 5 and 6 which are decentered with respect to theoptical axis is filled with a medium having a refractive index largerthan 1. In the example shown in FIG. 17, the image display apparatuscomprises a decentered prism 12 in which a space formed by threesurfaces 3, 4 and 6 which are decentered with respect to the opticalaxis is filled with a medium having a refractive index larger than 1. Ineach figure, reference numeral 1 denotes an observer's pupil; 2 denotesan observer's visual axis; 3 denotes a first surface of an ocularoptical system 12; 4 denotes a second surface of the ocular opticalsystem 12; 5 denotes a third surface of the ocular optical system 12; 6denotes a fourth surface of the ocular optical system 12; 7 denotes animage display device; 8 denotes a Fresnel; 9 denotes a line-of-sightdetecting optical system.; 10 denotes a line-of-sight detector; 11denotes an illuminating device; 12 denotes an ocular optical system(decentered prism); 13 and 14 denote second optical elements; 15 denotesan observer's eyeball; 16 denotes an optical filter; 17 denotes a linearmotor; 18 denotes projections provided on an optical element; and 19denotes a guide (rail) provided on a casing. In the examples shown inFIGS. 1 to 4, 5(b) to 11, 15 and 16, the actual path of light raysduring the observation of the electronic image is as follows: Light raysemitted from the electronic image of the image display device 7 enterthe ocular optical system 12 through the fourth surface 6, which is arefracting surface disposed to face the image display device 7. Theincident light rays are reflected toward the observer's pupil 1 by thethird surface 5, which is adjacent to the fourth surface 6 in the groupof two surfaces 4 and 5 located on the side of the ocular optical system12 that is remote from the observer's face. The reflected-light rays arereflected so as to travel away from the observer's pupil 1 by the firstsurface 3, which is disposed immediately in front of the observer'spupil 1. Then, the reflected light rays are reflected toward theobserver's pupil 1 by the second surface 4, which is disposedimmediately in front of the observer's pupil 1 in the group of twosurfaces 4 and 5 located on the side of the ocular optical system 12that is remote from the observer's face. The reflected light rays passthrough the first surface 3 and are projected into the observer'seyeball 15 with the observer's iris position or eyeball rolling centeras an exit pupil 1. In Example 17 shown in FIG. 17, light rays emittedfrom the electronic image of the image display device 7 enter the ocularoptical system 12 through the fourth surface 6, which is a refractingsurface disposed to face the image display device 7. The incident lightrays are reflected toward the observer's pupil 1 by that region (thirdsurface 5) of the second surface 4 which is adjacent to the fourthsurface 6. The second surface 4 also serves as the third surface 5,which is located on the side of the ocular optical system 12 that isremote from the observer's face. The reflected light rays are reflectedso as to travel away from the observer's pupil 1 by the first surface 3,which is disposed immediately in front of the observer's pupil 1. Then,the reflected light rays are reflected toward the observer's pupil 1 bythat region of the second surface 4 which is remote from the fourthsurface 6 and located on the side of the ocular optical system 12 thatis remote from the observer's face. The reflected light rays passthrough the first surface 3 and are projected into the observer'seyeball 15 with the observer's iris position or eyeball rolling centeras an exit pupil 1.

[0136] FIGS. 5(b) and 6 show examples of an image display apparatusaccording to the present invention that has a line-of-sight detectingdevice. The third surface 5, which is a reflecting surface disposed onthe outside world side of the ocular optical system 12, is set so that apart of the third surface 5 totally reflects light rays. The totallyreflecting portion of the third surface 5 can reflect light from theimage display device 7 without reflection coating. Therefore, noreflection coating is needed. The actual path of light rays during thedetection of the observer's line of sight is as follows: Illuminatinglight from the light source 11 passes through the third and firstsurfaces 5 and 3 of the ocular optical system 12 to illuminate theobserver's eyeball 15. Light rays reflected from the observer's eyeball15 enter the ocular optical system 12 through the first surface 3, whichis disposed immediately in front of the observer's pupil 1. The lightrays pass through a totally reflecting region provided in at least apart of the third surface 5, which is located on the side of the ocularoptical system 12 that is remote from the observer's face. The lightrays passing through the totally reflecting region of the third surface5 are led to the line-of-sight detector 10 through the line-of-sightdetecting optical system 9 to form an image of the observer's pupil 1.To reduce the effect of light from the electronic image or the like, itis possible to use an infrared light illuminating device as the lightsource 11 and an infrared light detector as the line-of-sight detector10. The position of the illuminating device 11 is not necessarilylimited to the illustrated position. The illuminating device 11 may bedisposed at any position, provided that the observer's eyeball 15 can beilluminated.

[0137]FIG. 18 is a sectional view of an arrangement in which aline-of-sight detecting device similar to the above, which comprises aline-of-sight detecting optical system 9, a line-of-sight detector 10,and a light source 11, is provided in an ocular optical system 12comprising three surfaces 3, 4 and 6 decentered with respect to theoptical axis as in Example 17. The actual path of light rays during thedetection of the observer's line of sight is similar to that in the caseof FIGS. 5(b) and 6; therefore, a description thereof is omitted.

[0138]FIG. 7 shows an example of an image display apparatus according tothe present invention that enables the electronic image and theexternal-scene image to be simultaneously observed through the-ocularoptical system 12. The actual path of light rays during the observationof the external-scene image is as follows: Light rays from an objectpoint in the external scene enter the ocular optical system 12 throughthe third surface 5, pass through the first surface 3 and are projectedinto the observer's eyeball 15 with the observer's iris position oreyeball rolling center as an exit pupil 1. It may be made easy for theobserver to view either or both of the-electronic image and theexternal-scene image by disposing a light-reducing filter or opticalelement 16 that controls the quantity of external light at the outsideworld side of the third surface 5. By moving the light-reducing filteror optical element 16 between the observation ranges α and β, it ispossible to control the quantity of light from either of the electronicimage and the external-scene image.

[0139] FIGS. 8(a), 8(b) and 8(c) and FIGS. 9(a), 9(b) and 9(c) showexamples of another image display apparatus according to the presentinvention that enables the external-scene image to be observed by movingthe ocular optical system 12. In the example shown in FIGS. 8(a), 8(b)and 8(c), the ocular optical system 12 is moved from the electronicimage observation position shown in FIG. 8(a) in the negative directionof the Y-axis relative to the observer's pupil 1 to reach theexternal-scene image observation position shown in FIG. 8(b). In theexample shown in FIGS. 9(a), 9(b) and 9(c),, the ocular optical system12 is rotated clockwise relative to-the observer's pupil 1 from theelectronic image observation position shown in FIG. 9(a) to reach theexternal-scene image observation position shown in FIG. 9(b). Therefore,in either case, the external scene can be observed in the direction ofthe observer's visual axis. Light rays from an object point in theexternal scene enter the ocular optical system 12 through the thirdsurface 5, pass through the first surface 3 and are projected into theobserver's eyeball with the observer's iris position or eyeball rollingcenter as an exit pupil 1. In the position shown in FIG. 8(b), theobserver can view the electronic image in a region below the observer'svisual axis 2. The direction of observation of the electronic imagediffers depending on the way in which the ocular optical system 12 isdisposed and the direction in which the ocular optical system 12 ismoved. Therefore, the observation of the electronic image may beperformed in any direction.

[0140] FIGS. 8(c) and 9(c) show examples of a mechanism for moving theocular optical system 12. In either case, the ocular optical system 12is moved along a guide (rail) 19 provided on the casing by a linearmotor 17 through projections 18 provided on the optical element. In thecase of FIG. 8(c), the guide (rail) 19 is rectilinear.

[0141] Therefore, the ocular optical system 12 is moved rectilinearly.In the case of FIG. 9(c), the guide (rail) 19 is arcuate. Therefore, theocular optical system 12 is rotated.

[0142] FIGS. 19(a), 19(b) and 19(c) show an example in which an ocularoptical system 12 comprising three surfaces 3, 4 and 5 decentered withrespect to the optical axis as in Example 17 is moved from theelectronic image observation position in the negative direction of theY-axis relative to the observer's pupil 1 to reach the external-sceneimage observation position as in the case of the example shown in FIGS.8(a), 8(b) and 8(c). The operation of this example is similar to that ofthe example shown in FIGS. 8(a), 8(b) and 8(c). Therefore, a descriptionthereof is omitted.

[0143] FIGS. 10 to 14 show examples of an image display apparatusaccording to the present invention in which a Fresnel lens 8 serving asan aberration correcting device is disposed in an optical path forobservation of the external-scene image. The actual path of light raysduring the observation of the external-scene image is as follows: Lightrays from an object point in the external scene pass through the Fresnellens 8 and enter the decentered prism 12 through the second surface 4.Then, the light rays pass through the first surface 3 and are projectedinto the observer's eyeball with the observer's iris position or eyeballrolling center as an exit pupil 1. In this arrangement, the Fresnel lens8 is only necessary to dispose at a predetermined position when theexternal scene is observed. When the observer does not-want to view theexternal scene, the Fresnel lens 8 is moved to another position by amoving mechanism, e.g. a mechanism for moving the Fresnel lens 8vertically, or a mechanism for rotating the Fresnel lens 8.Alternatively, the Fresnel lens 8 may be arranged to be detachable.

[0144] In the examples shown in FIGS. 10 to 14, those which are shown inFIGS. 10 and 11 are arranged as in the case of FIG. 1. That is, theocular optical system (decentered prism) 12 comprises four surfaces 3,4, 5 and 6 decentered with respect to the optical axis, and when theelectronic image is observed, light rays travel along the same path asin the case of FIG. 1. However, the ocular optical system shown in FIG.12 comprises a decentered prism 12 in which a space formed by threesurfaces 3, 4 and 6 decentered with respect to the optical axis isfilled with a medium having a refractive index larger than 1. The actualpath of light rays during the observation of the electronic image is asfollows: Light rays emitted from the electronic image of the imagedisplay device 7 enter the ocular optical system 12 through the fourthsurface (the third surface as counted in the sequence of surfaces) 6,which is a refracting surface disposed to face the image display device7. The incident light rays are reflected so as to travel away from theobserver's pupil 1 by the first surface 3, which is disposed immediatelyin front of the observer's pupil 1. The reflected light rays arereflected toward the observer's pupil 1 by the second surface 4, whichis disposed on the side of the ocular optical prism 12 that is remotefrom the observer's face. Then, the reflected light rays pass throughthe first surface 3 and are projected into the observer's eyeball 15with the observer's iris position or eyeball rolling center as an exitpupil 1.

[0145] The ocular optical system shown in FIG. 13 comprises a decenteredprism 12 in which a space formed by three surfaces 3, 4 and 6 decenteredwith respect to the optical axis is filled with a medium having arefractive index larger than 1. The actual path of light rays during theobservation of the electronic image is as follows: Light rays emittedfrom the electronic image of the image display device 7 enter the ocularoptical system 12 through the fourth surface (the third surface ascounted in the sequence of surfaces) 6, which is a refracting surfacedisposed to face the image display device 7. The incident light rays arereflected toward the observer's pupil 1 by the second surface 4, whichis disposed on the side of the ocular optical system 12 that is remotefrom the observer's face. The reflected light rays pass through thefirst surface 3 and are projected into the observer's eyeball 15 withthe observer's iris position or eyeball rolling center as an exit pupil1.

[0146] The ocular optical system 12 shown in FIG. 14 comprises adecentered prism 12 in which a space formed by two surfaces 3 and 4decentered with respect to the optical axis is filled with a mediumhaving a refractive index larger than 1. The actual path of-light raysduring the observation of the electronic image is as follows: Light raysemitted from the electronic image of the image display device 7 enterthe ocular optical system 12 through the first surface 3, which is arefracting surface disposed to face the image display device 7. Theincident light rays are reflected toward the observer's pupil 1 by thesecond surface 4, which is-located on the side of-the ocular opticalsystem 12 that is remote from the observer's face. The reflected lightrays pass through the first surface 3 and are projected into theobserver's eyeball 15 with the observer's iris position or eyeballrolling center as an exit pupil 1.

[0147]FIGS. 15 and 16 show examples of an image display apparatusaccording to the present invention which is adapted to observe theexternal scene through the first surface 3 of the ocular optical system12, which is located immediately in front of the observer's eyeball, andfurther through either of the second and third surfaces 4 and 5 of theocular optical system 12, which are outside world-side internallyreflecting surfaces. When the external scene is to be observed, secondoptical elements 13 and 14, which are adapted to cancel powers producedby the two surfaces 3 and 4 or 3 and 5 with respect to external light,are disposed in the optical paths for observation of the external-sceneimage. The actual path of light rays during the observation of theexternal-scene image is as follows: Light rays from an object point inthe external scene pass through a second optical element 13 or anothersecond optical element 14 and enter the decentered prism 12 through thesecond surface“4 or the third surface 5. Then, the light rays passthrough the first surface 3 and are projected into the observer'seyeball 15 with the observer's iris position or eyeball rolling centeras an exit pupil 1.

[0148] It should be noted that the present invention is not necessarilylimited to the optical systems shown in FIGS. 1 to 4 and 5(b) to 19 butmay also be applied to other known optical systems.

[0149] In the constituent parameters of each of the following examples,the rotationally symmetric aspherical configuration of each surface maybe given by the following equation on the assumption that the paraxialcurvature radius is denoted by R. The Z-axis is the axis of therotationally symmetric aspherical surface. $\begin{matrix}\begin{matrix}{Z = {{\left( {h^{2}/R} \right)/\left\lbrack {1 + \left\{ {1 - {\left( {1 + K} \right)\left( {h^{2}/R^{2}} \right)}} \right\}^{1/2}} \right\rbrack} +}} \\{{{A\quad h^{4}} + {B\quad h^{6}} + {Ch}^{8} + {{Dh}^{10}\cdots}}} \\{{~~~~~~~~~~~~~~~~~~~~~~~~}\left( {h^{2} = {x^{2} + y^{2}}} \right)}\end{matrix} & (a)\end{matrix}$

[0150] where Z is the amount of deviation from a plane tangent to theorigin of the surface configuration; K is the conical coefficient; andA, B, C and D are 4th-, 6th-, 8th- and 10th-order asphericalcoefficients, respectively.

[0151] The configuration of an anamorphic surface is defined by thefollowing equation. A straight line which passes through the origin ofthe surface configuration and which is perpendicular to the opticalsurface is defined as the axis of the anamorphic surface.$\begin{matrix}{Z = {\left( {{{CX} \cdot x^{2}} + {{CY} \cdot y^{2}}} \right)/\left\lbrack {1 + \left\{ {1 - {\left( {1 + K_{x}} \right){{CX}^{2} \cdot x^{2}}} -} \right.} \right.}} \\{\left. \left. {\left( {1 + K_{y}} \right){{CY}^{2} \cdot y^{2}}} \right\}^{1/2} \right\rbrack +} \\{{\sum\limits_{m = 1}^{\quad}\quad {R_{m}\left\{ {{\left( {1 - P_{m}} \right)x^{2}} + {\left( {1 + P_{m}} \right)y^{2}}} \right\}^{m + 1}}}}\end{matrix}$

[0152] Assuming that m=4 (polynomial of degree 4), for example, theequation, when expanded, may be given by: $\begin{matrix}\begin{matrix}{Z = {\left( {{{CX} \cdot x^{2}} + {{CY} \cdot y^{2}}} \right)/\left\lbrack {1 + \left\{ {1 - {\left( {1 + K_{x}} \right){{CX}^{2} \cdot x^{2}}} -} \right.} \right.}} \\{\left. {\left( {1 + K_{y}} \right){{CY}^{2} \cdot y^{2}}} \right\}^{1/2} +} \\{{{R_{1}\left\{ {{\left( {1 - P_{1}} \right)x^{2}} + {\left( {1 + P_{1}} \right)y^{2}}} \right\}^{2}} +}} \\{{{R_{2}\left\{ {{\left( {1 - P_{2}} \right)x^{2}} + {\left( {1 + P_{2}} \right)y^{2}}} \right\}^{3}} +}} \\{{{R_{3}\left\{ {{\left( {1 - P_{3}} \right)x^{2}} + {\left( {1 + P_{3}} \right)y^{2}}} \right\}^{4}} +}} \\{{R_{4}\left\{ {{\left( {1 - P_{4}} \right)x^{2}} + {\left( {1 + P_{4}} \right)y^{2}}} \right\}^{5}}}\end{matrix} & (b)\end{matrix}$

[0153] where Z is the amount of deviation from a plane tangent to theorigin of the surface configuration; CX is the curvature in the X-axisdirection; CY is the curvature in the Y-axis direction; K_(x) is theconical coefficient in the X-axis direction; K_(y) is the conicalcoefficient in the Y-axis direction; R_(m) is the rotationally symmetriccomponent of the aspherical surface term; and P_(m) is the rotationallyasymmetric component of the aspherical surface term. It should be notedthat in the constituent parameters of the examples (described later),the following parameters are employed:

[0154] R_(x): the radius of curvature in the X-axis direction

[0155] R_(y): the radius of curvature in the Y-axis direction

[0156] The curvature radii are related to the curvatures CX and CY asfollows:

R _(x)=1/CX, R _(y)=1/CY

[0157] The configuration of a three-dimensional surface is defined bythe following equation. The Z-axis of the defining equation is the axisof the three-dimensional surface.$Z = {\sum\limits_{n = 0}^{k}\quad {\sum\limits_{m = k}^{n}\quad {C_{n\quad m}X^{n}Y^{n - m}}}}$

[0158] Assuming that k=7 (polynomial of degree 7), for example, athree-dimensional surface is expressed by an expanded form of the aboveequation as follows: $\begin{matrix}\begin{matrix}{Z = {C_{2} +}} \\{{{C_{3}Y} + {C_{4}X} +}} \\{{{C_{5}Y^{2}} + {C_{6}{YX}} + {C_{7}X^{2}} +}} \\{{{C_{8}Y^{3}} + {C_{9}Y^{2}X} + {C_{10}{YX}^{2}} + {C_{11}X^{3}} +}} \\{{{C_{12}Y^{4}} + {C_{13}Y^{3}X} + {C_{14}Y^{2}X^{2}} + {C_{15}{YX}^{3}} + {C_{16}X^{4}} +}} \\{{{C_{17}Y^{5}} + {C_{18}Y^{4}X} + {C_{19}Y^{3}X^{2}} + {C_{20}Y^{2}X^{3}} + {C_{21}{YX}^{4}} + {C_{22}X^{5}} +}} \\{{{C_{23}Y^{6}} + {C_{24}Y^{5}X} + {C_{25}Y^{4}X^{2}} + {C_{26}Y^{3}X^{3}} + {C_{27}Y^{2}X^{4}} + {C_{28}{YX}^{5}} +}} \\{{{C_{29}X^{6}} + {C_{30}Y^{7}} + {C_{31}Y^{6}X} + {C_{32}Y^{5}X^{2}} + {C_{33}Y^{4}X^{3}} + {C_{34}Y^{3}X^{4}} +}} \\{{{C_{35}Y^{2}X^{5}} + {C_{36}{YX}^{6}} + {C_{37}X^{7}}}}\end{matrix} & (c)\end{matrix}$

[0159] In the examples of the present invention, each ocular opticalsystem is designed as an optical system symmetric with respect to theX-axis direction. Therefore, the coefficients of the terms withodd-numbered powers of X are set equal to zero [in the above equation(c), C₄, C₆, C₉, . . . =0].

[0160] In the constituent parameters (shown later), those termsconcerning aspherical surfaces for which no data is shown are zero. Therefractive index is expressed by the refractive index for the spectrald-line (wavelength: 58.7.56 nanometers). Lengths are given inmillimeters.

[0161] FIGS. 1 to 4 and 5(b) to 17 are sectional views of Examples 1 to4 and 5 to 17 taken along the YZ-plane containing the optical axis 2. InExamples 1 to 11, 13 and 14, the observation field angles are asfollows: The horizontal field angle is 30.0°, and the vertical fieldangle is 22.72°. In Example 12, the observation field angles are asfollows: The horizontal field angle is 40.0°, and the vertical fieldangle is 30.53°. In Examples 15 and 16, the observation field anglesare-as follows: The horizontal field angle is 35.0°, and the verticalfield angle is 26.60°. In Examples 1 to 16, the pupil diameter is 4millimeters.

[0162] The constituent parameters and the values of the conditions inthe above-described Examples 1 to 6, 9 to 14 and 17 are shown below. Theconstituent parameters of Examples 7 and 8 are the same as those ofExample 3; therefore a description thereof is omitted. The constituentparameters of Examples 10 and 11 during the observation of the imagedisplay device are the same as those of Example 5. Therefore, theconstituent parameters during the observation of the external scene areshown for Examples 10 and 11. The constituent parameters of Example 12during the observation of the image display device are shown under“Example 12(1)”. The constituent parameters of Example 12 during theobservation of the external scene are shown under “Example 12(2)”. Itshould be noted that in the table below, “ASPH” denotes an asphericalsurface; “ANAM” denotes an anamorphic surface; “SF” denotes a surface;and “REFL” denotes a reflecting surface.

EXAMPLE 1

[0163] Refractive Surface Radius of Surface index Abbe's No. No.curvature separation (Displacement) (Tilt angle) 1 ∞(Pupil) 2 ASPH ∞ 1.5254  56.25 (1ST SF) K   0.0000 Y 18.114 θ  4.44° A   0.0000 Z 37.091B   0.0000 C   1.1599 × 10⁻¹³ D   4.4930 × 10⁻¹⁶ 3 ANAM R_(y) −142.541 1.5254  56.25 (2ND SF) R_(x) −122.057 Y  3.041 θ −17.79° (REFL) K_(y) −5.4587 Z 52.132 K_(x)  −0.2658 R₁  −5.0900 × 10⁻¹⁰ R₂   3.0528 × 10⁻¹⁰R₃   6.2600 × 10⁻¹³ R₄   4.9434 × 10⁻¹⁵ P₁  −1.1948 × 10⁺¹ P₂   2.3791 ×10⁻¹ P₃   4.8713 × 10⁻¹ P₄   3.3074 × 10⁻¹ 4 ASPH ∞  1.5254  56.25 (1STSF) K   0.0000 Y 18.114 θ  4.44° (REFL) A   0.0000 Z 37.091 B   0.0000 C  1.1599 × 10⁻¹³ D   4.4930 × 10⁻¹⁶ 5 ∞  1.5254  56.25 (3RD SF) Y 18.114θ  4.44° (REFL) Z 53.502 6 ANAM R_(y)  47.391 Y 41.220 θ −55.16° (4THSF) R_(x)  86.005 Z 47.787 K_(y)   1.9910 K_(x)  −0.1607 R₁   1.1694 ×10⁻⁷ R₂  −2.2052 × 10⁻¹⁰ R₃  −1.8410 × 10⁻¹¹ R₄  −4.2076 × 10⁻¹⁴ P₁ −6.2804 P₂  −4.0710 P₃   4.2066 × 10⁻¹ P₄   5.1697 × 10⁻¹ 7 ∞ Y 40.079θ −25.63° (Image display plane) Z 38.041 (1) θ_(r3) = 43.85° (3) Φ_(t1)(yz) = 0 (1/mm) Φ_(t1) (xz) = 0 (1/mm)

EXAMPLE 2

[0164] Refractive Surface Radius of Surface index Abbe's No. No.curvature separation (Displacement) (Tilt angle) 1 ∞(Pupil) 2Three-dimensional surface(1)  1.5000  55.55 (1ST SF) Y  8.738 θ  −0.43°Z 38.294 3 Three-dimensional surface(2)  1.5000  55.55 (2ND SF) Y  0.000θ −26.39° (REFL) Z 47.232 4 Three-dimensional surface(1)  1.5000  55.55(1ST SF) Y  8.738 θ  −0.43° (REFL) Z 38.294 5 Three-dimensionalsurface(3)  1.5000  55.55 (3RD SF) Y 28.900 θ  4.00° (REFL) Z 51.503 6Three-dimensional surface(4)  1.5000  55.55 (4TH SF) Y 37.094 θ −42.91°Z 43.146 7 ∞ Y 39.222 θ −39.45° (Image display plane) Z 41.032Three-dimensional surface(1) C₅ −4.3507 × 10⁻⁴ C₇ −8.3810 × 10⁻³ C₈−7.2046 × 10⁻⁵ C₁₀ −1.6070 × 10⁻⁴ C₁₂ −5.7849 × 10⁻⁷ C₁₄ −7.6285 × 10⁻⁷C₁₆  2.6344 × 10⁻⁶ C₁₇ −7.4711 × 10⁻⁹ C₁₉ −1.9337 × 10⁻⁸ C₂₁  9.3990 ×10⁻⁸ Three-dimensional surface(2) C₅ −4.4979 × 10⁻³ C₇ −8.5757 × 10⁻³ C₈−6.4211 × 10⁻⁵ C₁₀ −3.1176 × 10⁻⁵ C₁₂  1.3495 × 10⁻⁶ C₁₄  4.8979 × 10⁻⁸C₁₆ −2.4100 × 10⁻⁸ C₁₇ −4.2204 × 10⁻⁸ C₁₉ −3.8212 × 10⁻⁸ C₂₁ −9.1979 ×10⁻⁹ Three-dimensional surface(3) C₅ −4.6997 × 10⁻⁴ C₇ −3.2125 × 10⁻³ C₈−8.6078 × 10⁻⁵ C₁₀ −1.0181 × 10⁻⁴ C₁₂ −2.7246 × 10⁻⁶ C₁₄  2.7277 × 10⁻⁶C₁₆  3.7002 × 10⁻⁶ C₁₇ −3.1103 × 10⁻⁸ C₁₉  1.0092 × 10⁻⁸ C₂₁  2.0208 ×10⁻⁷ Three-dimensional surface(4) C₅  3.6987 × 10⁻³ C₇  8.3763 × 10⁻³ C₈−8.9771 × 10⁻⁴ C₁₀  5.0916 × 10⁻⁶ C₁₂ −5.7678 × 10⁻⁵ C₁₄  4.5123 × 10⁻⁷C₁₆ −2.3865 × 10⁻⁶ (1) θ_(r3) = 48.44° (3) Φ_(t1) (yz) = −0.0037 (1/mm)Φ_(t1) (xz) = −0.0072 (1/mm)

EXAMPLE 3

[0165] Refractive Surface Radius of Surface index Abbe's No. No.curvature separation (Displacement) (Tilt angle) 1 ∞(Pupil) 2 ∞  1.5254 56.25 (1ST SF) Y −27.000 θ  0.00° Z  37.395 3 ANAM R_(y) −143.929 1.5254  56.25 (2ND SF) R_(x) −123.293 Y −14.868 θ −28.80° (REFL) K_(y)  0.3713 Z  42.871 K_(x)  −1.9942 R₁   2.1403 × 10⁻⁸ R₂   9.6413 × 10⁻¹³R₃   6.3684 × 10⁻¹⁴ R₄  −1.2452 × 10⁻¹⁷ P₁  −3.9989 × 10⁻³ P₂  −3.0463P₃   2.5677 × 10 − 1 P₄   4.2810 × 10 − 1 4 ∞  1.5254  56.25 (1ST SF) Y−27.000 θ  0.00° (REFL) Z  37.395 5 ∞  1.5254  56.25 (3RD SF) Y  0.079 θ 0.00° (REFL) Z  53.539 6 ANAM R_(y)  39.861 Y  44.498 θ −66.77° (4THSF) R_(x)  62.319 Z  51.066 K_(y)   1.5656 K_(x)   4.2425 R₁   3.9064 ×10⁻⁶ R₂   5.0520 × 10⁻¹⁰ R₃   4.9921 × 10⁻¹³ R₄  −6.6158 × 10⁻¹⁵ P₁ −1.5408 × 10⁻¹ P₂   4.0979 P₃   1.6631 P₄   1.0506 7 ∞ Y  42.800 θ−21.40° (Image display plane) Z  38.475 (1) θ_(r3) = 44.53° (3) Φ_(t1)(yz) = 0 (1/mm) Φ_(t1) (xz) = 0 (1/mm)

EXAMPLE 4

[0166] Refractive Surface Radius of Surface index Abbe's No. No.curvature separation (Displacement) (Tilt angle) 1 ∞(Pupil) 2 ANAM R_(y)−242.348  1.5254  56.25 (1ST SF) R_(x) −159.768 Y  5.082 θ  −3.86° K_(y) 12.1104 Z 32.396 K_(x)   4.7358 R₁  −2.5719 × 10⁻¹⁰ R₂  −4.9792 × 10⁻¹²R₃   8.8695 × 10⁻¹³ R₄   6.7191 × 10⁻²⁰ P₁  −1.8150 × 10⁺¹ P₂  −4.7838P₃  −1.2978 P₄  −7.1284 3 ANAM R_(y) −119.562  1.5254  56.25 (2ND SF)R_(x)  −98.451 Y 27.149 θ −11.26° (REFL) K_(y)  −0.1186 Z 52.500 K_(x)  0.7866 R₁  −1.6969 × 10⁻⁹ R₂  −6.2266 × 10⁻¹¹ R₃   8.5459 × 10⁻¹⁶ R₄  8.0998 × 10⁻¹⁶ P₁  −1.8331 P₂  −4.9789 × 10⁻¹ P₃  −2.3604 P₄  −9.6450× 10⁻¹ 4 ANAM R_(y) −242.348  1.5254  56.25 (1ST SF) R_(x) −159.768 Y 5.082 θ  −3.86° (REFL) K_(y)  12.1104 Z 32.396 K_(x)   4.7358 R₁ −2.5719 × 10⁻¹⁰ R₂  −4.9792 × 10⁻¹² R₃   8.8695 × 10⁻¹³ R₄   6.7191 ×10⁻²⁰ P₁  −1.8150 × 10⁺¹ P₂  −4.7838 P₃  −1.2978 P₄  −7.1284 5 ANAMR_(y) −179.007  1.5254  56.25 (3RD SF) R_(x) −231.111 Y 27.820 θ  1.74°(REFL) K_(y)   2.2288 Z 47.835 K_(x)  −72.7188 R₁   6.1912 × 10⁻⁸ R₂ −9.4470 × 10⁻¹³ R₃   2.8064 × 10⁻¹⁵ R₄   2.0069 × 10⁻¹⁸ P₁   2.0705 ×10⁻² P₂   6.7667 P₃  −5.5003 P₄  −4.0534 6 ANAM R_(y)  72.293 Y 42.329 θ−42.24° (4TH SF) R_(x)  39.167 Z 43.924 K_(y)  −1.0213 K_(x)  −7.8305 R₁ −7.5404 × 10⁻⁷ R₂  −5.8510 × 10⁻¹⁰ R₃   5.8345 × 10⁻¹³ R₄   1.5291 ×10⁻¹⁵ P₁  −1.2077 × 10⁻¹ P₂   2.1174 × 10⁻² P₃   2.9220 × 10⁻¹ P₄ −1.4519 7 ∞ Y 42.878 θ −19.36° (Image display plane) Z 30.211 (1)θ_(r3) = 42.75° (3) Φ_(t1) (yz) = 0.0008 (1/mm) Φ_(t1) (xz) = −0.001(1/mm)

EXAMPLE 5

[0167] Refractive Surface Radius of Surface index Abbe's No. No.curvature separation (Displacement) (Tilt angle) 1 ∞(Pupil) 2  −104.851 1.5254  56.25 (1ST SF) Y  2.540 θ  −7.02° Z  33.527 3 ANAM R_(y) −54.751  1.5254  56.25 (2ND SF) R_(x)  −62.006 Y −19.747 θ −48.21°(REFL) K_(y)   −1.3614 Z  30.166 K_(x)   0.1944 R₁   2.4430 × 10⁻¹⁰ R₂  −1.1189 × 10⁻¹⁰ R₃   −2.4892 × 10⁻¹⁶ R₄   1.9084 × 10⁻²² P₁   −2.7674× 10⁺¹ P₂   5.3845 × 10⁻¹ P₃   −4.1468 P₄   1.0048 × 10⁺¹ 4  −104.851 1.5254  56.25 (1ST SF) Y  2.540 θ  −7.02° (REFL) Z  33.527 5 ANAM R_(y)−8201.935  1.5254  56.25 (3RD SF) R_(x)  1243.857 Y −37.497 θ  4.95°(REFL) K_(y)   0.0000 Z  53.061 K_(x)   0.0000 R₁   9.7227 × 10⁻⁸ R₂  1.2246 × 10⁻¹² R₃   −1.5956 × 10⁻¹⁶ R₄   6.7677 × 10⁻²¹ P₁   8.5858 ×10⁻¹ P₂   −4.4664 P₃   1.9991 P₄   2.2019 6   46.674 Y  28.160 θ −29.19°(4TH SF) Z  36.643 7 ∞ Y  31.793 θ −26.09° (Image display plane) Z 35.135 (1) θ_(r3) = 36.51°

EXAMPLE 6

[0168] Refractive Surface Radius of Surface index Abbe's No. Nocurvature separation (Displacement) (Tilt angle) 1 ∞(Pupil) 2 ∞ Y  0.000θ  20.00° (Hypothetic plane) Z  0.000 3 ∞  1.5254  56.25 (1ST SF) (fromhypothetic plane) Y  0.000 θ  0.00° Z  40.495 4 ANAM R_(y) −146.661 1.5254  56.25 (2ND SF) R_(x) −131.067 (from hypothetic plane) (REFL)K_(y)  −0.1158 Y −23.006 θ −32.35° K_(x)  −0.6570 Z  49.040 R₁   1.4710× 10⁻⁸ R₂   2.4181 × 10⁻¹⁰ R₃   8.0445 × 10⁻¹⁴ R₄  −1.0655 × 10⁻¹⁶ P₁ −6.7968 × 10⁻¹ P₂   1.1524 × 10⁻² P₃   9.6151 × 10⁻¹ P₄   5.6260 × 10⁻¹5 ∞  1.5254  56.25 (1ST SF) (from hypothetic plane) (REFL) Y  0.000 θ 0.00° Z  40.495 6 ∞  1.5254  56.25 (3RD SF) (from hypothetic plane)(REFL) Y  0.000 θ  0.00° Z  56.475 7 ANAM R_(y)  70.881 (from hypotheticplane) (4TH SF) R_(x)  99.816 Y  30.811 θ −80.98° K_(y)   6.0488 Z 62.245 K_(x)   7.1389 R₁   1.8385 × 10⁻⁵ R₂   1.8499 × 10⁻¹⁰ R₃ −3.4116 × 10⁻¹² R₄  −7.2747 × 10⁻¹⁵ P₁   3.2623 × 10⁻¹ P₂   3.8697 P₃  9.0201 × 10⁻¹ P₄   1.1638 × 10⁻¹ 8 ∞ Y  40.634 θ  −2.65° (Imagedisplay plane) Z  30.403 (1) θ_(r3) = 46.70° (3) Φ_(t1) (yz) = 0 (1/mm)Φ_(t1) (xz) = 0 (1/mm)

EXAMPLE 9

[0169] Refractive Surface Radius of Surface index Abbe' s No. No.curvature separation (Displacement) (Tilt angle) 1 ∞(Pupil) 2 ∞ Y  0.000θ  15.00° (Hypothetic plane) Z  0.000 3 −221.433  1.5254  56.25 (1ST SF)(from hypothetic plane) Y  0.000 θ  0.00° Z  38.879 4 −106.803  1.5254 56.25 (2ND SF) (from hypothetic plane) (REFL) Y −16.310 θ −30.86° Z 48.157 5 −221.433  1.5254  56.25 (1ST SF) (from hypothetic plane)(REFL) Y  0.000 θ  0.00° Z  38.879 6 −208.964  1.5254  56.25 (3RD SF)(from hypothetic plane) (REFL) Y  0.000 θ  0.00° Z  55.417 7  154.685(from hypothetic plane) (4TH SF) Y  22.393 θ −20.71° Z  41.581 8 ∞ Y 39.534 θ  −5.00° (Image display plane) Z  27.732 (1) θ_(r3) = 41.68°(3) Φ_(t1) (yz) = 0.00024 (1/mm) Φ_(t1) (xz) = 0.00024 (1/mm)

EXAMPLE 10

[0170] Refractive Surface Radius of Surface index Abbe's No. No.curvature separation (Displacement) (Tilt angle) 1 ∞(Pupil) 2 −104.851 1.5254  56.25 (1ST SF) Y  2.540 θ  −7.02° Z  33.527 3 ANAM R_(y) −54.751 Y −19.747 θ −48.21° (2ND SF) R_(x)  −62.006 Z  30.167 (REFL)K_(y)  −1.3614 K_(x)   0.1944 R₁   2.4430 × 10⁻¹⁰ R₂  −1.1189 × 10⁻¹⁰ R₃ −2.4892 × 10⁻¹⁶ R₄   1.9084 × 10⁻²² P₁  −2.7674 × 10⁺¹ P₂   5.3845 ×10⁻¹ P₃  −4.1468 P₄   1.0048 × 10⁺¹ 4 ∞ 2.000  1.4922  57.50(Fresnel-lens's first surface) Y  45.000 θ  0.00° Z  51.527 5 ∞(Fresnel-lens's second surface) K   0.0000 A   2.0658 × 10⁻⁶ B  −4.2780× 10⁻¹⁰ C   3.2196 × 10⁻¹⁴ D   2.1256 × 10⁻¹⁸

EXAMPLE 11

[0171] Refractive Surface Radius of Surface index Abbe's No. No.curvature separation (Displacement) (Tilt angle) 1 ∞(Pupil) 2 −104.851 1.5254  56.25 (1ST SF) Y  2.540 θ  −7.02° Z  33.527 3 ANAM R_(y) −54.751 Y −19.747 θ −48.21° (2ND SF) R_(x)  −62.006 Z  30.166 (REFL)K_(y)  −1.3614 K_(x)   0.1944 R₁   2.4430 × 10⁻¹⁰ R₂  −1.1189 × 10⁻¹⁰ R₃ −2.4892 × 10⁻¹⁶ R₄   1.9084 × 10⁻²² P₁  −2.7674 × 10⁺¹ P₂   5.3845 ×10⁻¹ P₃  −4.1468 P₄   1.0048 × 10⁺¹ 4 ∞ 2.000  1.4922  57.50(Fresnel-lens's first surface) Y  20.000 θ −22.00° Z  53.527 5 ∞(Fresnel-lens's second surface) K   0.0000 A   4.4111 × 10⁻⁵ B  −1.0534× 10⁻⁷ C   1.1649 × 10⁻¹⁰ D  −4.9416 × 10⁻¹⁴ °

EXAMPLE 12 (1)

[0172] Refractive Surface Radius of Surface index Abbe's No. No.curvature separation (Displacement) (Tilt angle) 1 ∞(Pupil) 2Three-dimensional surface(1)  1.5254  56.25 (1ST SF) Y 13.983 θ  9.46° Z33.974 3 Three-dimensional surface(2)  1.5254  56.25 (2ND SF) Y  4.596 θ−15.22° (REFL) Z 49.231 4 Three-dimensional surface(1)  1.5254  56.25(1ST SF) Y 13.983 θ  9.46° (REFL) Z 33.974 5 Three-dimensionalsurface(3) Y 27.094 θ  79.39° (3RD SF) Z 35.215 6 ∞ Y 29.266 θ  46.34°(Image display plane) Z 46.318 Three-dimensional surface(1) C₅ −2.6152 ×10⁻³ C₇ −3.9706 × 10⁻³ C₈ −7.5434 × 10⁻⁵ C₁₀ −1.5120 × 10⁻⁶ C₁₂  2.6572× 10⁻⁷ C₁₄  1.3359 × 10⁻⁶ C₁₆  1.7946 × 10⁻⁷ C₁₇ −2.9881 × 10⁻⁹ C₁₉−3.0362 × 10⁻⁹ C₂₁ −2.0258 × 10⁻⁷ C₂₃ −3.8978 × 10⁻¹⁰ C₂₅  1.4986 × 10⁻⁹C₂₇ −3.8974 × 10⁻⁹ C₂₉ −2.5335 × 10⁻⁹ C₃₀  4.3101 × 10⁻¹² C₃₂ −1.4923 ×10⁻¹¹ C₃₄  7.6026 × 10⁻¹¹ C₃₆ −4.2410 × 10⁻¹¹ Three-dimensionalsurface(2) C₅ −6.2524 × 10⁻³ C₇ −7.5944 × 10⁻³ C₈ −1.0605 × 10⁻⁵ C₁₀ 9.3276 × 10⁻⁶ C₁₂  8.3882 × 10⁻⁷ C₁₄ −5.6861 × 10⁻⁷ C₁₆ −4.9904 × 10⁻⁷C₁₇ −2.0403 × 10⁻¹⁰ C₁₉ −8.0184 × 10⁻⁹ C₂₁ −4.4196 × 10⁻⁸ C₂₃  4.4149 ×10⁻¹⁰ C₂₅  3.8170 × 10⁻¹⁰ C₂₇  8.4970 × 10⁻¹¹ C₂₉ −2.8006 × 10⁻¹⁰ C₃₀ 1.3964 × 10⁻¹² C₃₂ −1.7677 × 10⁻¹⁰ C₃₄  3.3220 × 10⁻¹² C₃₆  6.9401 ×10^(−l2) Three-dimensional surface(3) C₅ −1.2118 × 10⁻² C₇ −3.7062 ×10⁻³ C₈ −1.2290 × 10⁻⁴ C₁₀  9.9763 × 10⁻⁴ C₁₂ −8.0746 × 10⁻⁵ C₁₄ −3.8939× 10⁻⁵ C₁₆  2.6861 × 10⁻⁵ C₁₇ −1.7720 × 10⁻⁶ C₁₉ −3.4243 × 10⁻⁶ C₂₁−3.5310 × 10⁻⁷ C₂₃  1.2185 × 10⁻⁷ C₂₅  1.0019 × 10⁻⁷ C₂₇  1.4838 × 10⁻⁷C₂₉ −5.3531 × 10⁻⁸

EXAMPLE 12 (2)

[0173] Refractive Surface Radius of Surface index Abbe's No. No.curvature separation (Displacement) (Tilt angle) 1 ∞(Pupil) 2Three-dimensional 1.5254 56.25 surface(1) (1ST SF) Y 13.983 θ 9.46° Z33.974 3 Three-dimensional Y 4.596 θ −15.22° surface(2) (2ND SF) Z49.231 4 ∞ 2.000 1.4922 57.50 (Fresnel-lens's first surface) Y 45.982 θ−18.17° Z 65.000 5 ∞ (Fresnel-lens's second surface) K  0.0000 A  3.9372× 10⁻⁶ B −1.6979 × 10⁻⁹ C  4.2377 × 10⁻¹³ D −4.1829 × 10⁻¹⁷Three-dimensional surface(1) C₅  −2.6152 × 10⁻³ C₇  −3.9706 × 10⁻³ C₈ −7.5434 × 10⁻⁵ C₁₀ −1.5120 × 10⁻⁶ C₁₂  2.6572 × 10⁻⁷ C₁₄  1.3359 × 10⁻⁶C₁₆  1.7946 × 10⁻⁷ C₁₇ −2.9881 × 10⁻⁹ C₁₇ −3.0362 × 10⁻⁹ C₂₁ −2.0258 ×10⁻⁷ C₂₃ −3.8978 × 10⁻¹⁰ C₂₅  1.4986 × 10⁻⁹ C₂₇ −3.8974 × 10⁻⁹ C₂₉−2.5335 × 10⁻⁹ C₃₀  4.3101 × 10⁻¹² C₃₂ −1.4923 × 10⁻¹¹ C₃₄  7.6026 ×10⁻¹¹ C₃₆ −4.2410 × 10⁻¹¹ Three-dimensional surface(2) C₅  −6.2524 ×10⁻³ C₇  −7.5944 × 10⁻³ C₈  1.0605 × 10⁻⁵ C₁₀  9.3276 × 10⁻⁶ C₂  8.3882× 10⁻⁷ C₁₄ −5.6861 × ¹⁰ ⁻⁷ C₁₆ −4.9904 × 10⁻⁷ C₁₇ −2.0403 × 10⁻¹⁰ C₁₉−8.0184 × 10⁻⁹ C₂₁  4.4196 × 10⁻⁸ C₂₃  4.4149 × 10⁻¹⁰ C₂₅  3.8170 ×10⁻¹⁰ C₂₇  8.4970 × 10⁻¹¹ C₂₉ −2.8006 × 10⁻¹⁰ C₃₀  1.3964 × 10⁻¹² C₃₂−1.7677 × 10⁻¹⁰ C₃₄  3.3220 × 10⁻¹² C₃₆  6.9401 × 10⁻¹²

EXAMPLE 13

[0174] Refractive Surface Radius of Surface index Abbe's No. No.curvature separation (Displacement) (Tilt angle) 1 ∞(Pupil) 2Three-dimensional 1.5163 64.15 surface(1) (1ST SF) Y 0.000 θ 24.79° Z35.567 3 Three-dimensional 1.5163 64.15 surface(2) (2ND SF) Y 5.402 θ−9.11° (REFL) Z 70.723 4 Three-dimensional Y 21.138 θ −25.12° surface(3)(3RD SF) Z 39.783 5 ∞ Y 23.963 θ −11.11° (Image display plane) Z 34.441Three-dimensional surface(1) C₅  6.8620 × 10⁻³ C₇  7.4153 × 10⁻³ C₈ 5.9417 × 10⁻⁵ C₁₀  2.9033 × 10⁻⁵ C₁₂ −4.6823 × 10⁻⁷ C₁₄  3.8805 × 10⁻⁶C₁₆  5.0284 × 10⁻⁷ C₁₇  2.3906 × 10⁻⁸ C₁₉  7.1030 × 10−8 C₂₁  2.8323 ×10⁻⁸ Three-dimensional surface(2) C₅  −3.7101 × 10⁻³ C₇  −4.1036 × 10−3C₈  4.2896 × 10−6 C₁₀ −8.4314 × 10⁻⁶ C₁₂ −8.1477 × 10⁻⁸ C₁₄  1.1846 ×10⁻⁶ C₁₆  2.8608 × 10⁻⁷ C₁₇  8.8332 × 10⁻⁹ C₁₉  3.2284 × 10⁻⁸ C₂₁ 1.2745 × 10⁻⁸ Three-dimensional surface(3) C₅  1.5613 × 10⁻² C₇  1.5901× 10⁻² C₈  3.8223 × 10⁻⁴ C₁₀ −5.9546 × 10⁻⁵ C₁₂ −5.8106 × 10⁻⁵ C₁₄−4.2859 × 10⁻⁵ C₁₆ −2.2163 × 10⁻⁵ C₁₇  1.1940 × 10⁻⁶ C₁₉  2.0760 × 10⁻⁶C₂₁  1.0626 × 10⁻⁶

EXAMPLE 14

[0175] Refractive Surface Radius of Surface index Abbe's No. No.curvature separation (Displacement) (Tilt angle) 1 ∞(Pupil) 2Three-dimensional 1.5163 64.15 surface(1) (1ST SF) Y −10.123 θ 20.33° Z43.489 3 Three-dimensional 1.5163 64.15 surface(2) (2ND SF) Y 1.103 θ−10.31° (REFL) Z 65.000 4 Three-dimensional Y −10.123 θ 20.33°surface(1) (1ST SF) Z 43.489 5 ∞ Y 17.608 θ −13.99° (Image displayplane) Z 30.846 Three-dimensional surface(1) C₅  1.0401 × 10⁻² C₇ 8.6572 × 10⁻³ C₈  9.8267 × 10⁻⁵ C₁₀  2.0456 × 10⁻⁴ C₁₂ −9.4226 × 10⁻⁶C₁₄  1.6262 × 10⁻⁶ C₁₆  4.0506 × 10⁻⁶ C₁₇  3.2669 × 10⁻⁷ C₁₉  2.1072 ×¹⁰ ⁻⁷ C₂₁  1.5355 × 10⁻⁷ Three-dimensional surface(2) C₅  −2.5798 × 10⁻³C₇  −3.0708 × 10⁻³ C₈  −3.2024 × 10⁻⁶ C₁₀ −3.3909 × 10⁻⁶ C₁₂  2.9430 ×10⁻⁶ C₁₄  4.3427 × ⁻⁸ C₁₆  3.4981 × 10⁻⁶ C₁₇ −2.8763 × ⁻⁸ C₁₉  4.0895 ×⁻⁸ C₂₁  5.4666 × 10⁻⁸

EXAMPLE 17

[0176] Refractive Surface Radius of Surface index Abbe's No. No.curvature separation (Displacement) (Tilt angle) 1 ∞(Pupil) 2Three-dimensional 1.5000 55.55 surface(1) (1ST SF) Y 18.958 θ 7.69° Z30.730 3 Three-dimensional 1.5000 55.55 surface(2) (2ND SF) Y 9.165 θ−13.84° (REFL) Z 48.107 4 Three-dimensional 1.5000 55.55 surface(1) (1STSF) Y 18.958 θ 7.69° (REFL) Z 30.730 5 Three-dimensional 1.5000 55.55surface(2) (2ND SF) Y 9.165 θ −13.84° (REFL) Z 48.107 6Three-dimensional 1.5000 55.55 surface(3) (4TH SF) Y 34.128 θ −31.50° Z30.758 7 ∞ Y 47.350 θ −34.92° (Image display plane) Z 35.893Three-dimensional surface(1) C₅  −4.9463 × 10⁻³ C₇  −3.4912 × 10⁻³ C₈ 6.9477 × 10⁻⁵ C₁₀  1.7114 × 10⁻⁴ C₁₂  1.0830 × 10⁻⁶ C₁₄ −2.2541 × 10⁻⁷C₁₆  4.5743 × 10⁻⁶ C₁₇  6.1581 × 10⁻⁸ C₁₉  4.7667 × 10⁻⁸ C₂₁ −1.9359 ×10⁻⁷ C₂₃ −1.3103 × 10⁻¹⁰ C₂₅ −7.7572 × 10⁻¹⁰ C₂₇  7.0783 × 10⁻¹⁰ C₂₉ 5.3774 × 10⁻⁹ C₃₀  4.7726 × 10⁻¹² C₃₂  1.3699 × 10⁻¹¹ C₃₄  7.4217 ×10⁻¹¹ C₃₆ −1.3460 × 10⁻¹⁰ Three-dimensional surface(2) C₅  −5.9243 ×10⁻³ C₇  −5.4509 × 10⁻³ C₈  3.4016 × 10⁻⁵ C₁₀  7.9633 × 10⁻⁵ C₁₂ −4.1470× 10⁻⁷ C₁₄  1.0233 × 10⁻⁶ C₁₆  2.6471 × 10⁻⁶ C₁₇  2.3016 × 10⁻⁹ C₁₉ 3.3134 × 10⁻⁸ C₂₁ −1.6456 × 10⁻⁸ C₂₃ −1.3255 × 10⁻¹⁰ C₂₅ −4.9215 ×10⁻¹⁰ C₂₇ −3.3070 × 10⁻¹⁰ C₂₉  4.1802 × 10⁻⁹ Three-dimensionalsurface(3) C₅  7.9798 × 10⁻³ C₇  1.7546 × 10⁻² C₈  −1.1020 × 10⁻⁴ C₁₀ 9.4392 × 10⁻⁴ C₁₂ −3.9282 × 10⁻⁶ C₁₄ −7.3326 × 10⁻⁶ C₁₆ −1.4273 × 10⁻⁵(1) θ_(r3) = 46.48°

[0177] Although in the above-described examples the optical systems areconstructed by using aspherical surfaces, anamorphic surfaces andthree-dimensional surfaces defined by the above equations (a), (b) and(c), it is also possible to use surface configurations expressed byZernike polynomials as defined by the following equation (d) andthree-dimensional surfaces symmetric with respect to the X-axisdirection as defined by the following equation (e). That is, curvedsurfaces expressed by any defining equations can be used.

[0178] Plane-symmetry three-dimensional surfaces may also be defined byZernike polynomials. That is, the configuration of a plane-symmetrythree-dimensional surface may be defined by the following equation (d).The Z-axis of the defining equation (d) is the axis of Zernikepolynomial. $\begin{matrix}{{X = {R \times {\cos (A)}}}{Y = {R \times {\sin (A)}}}{Z = {D_{2} + {D_{3}R\quad {\cos (A)}} + {D_{4}R\quad {\sin (A)}} + {D_{5}R^{2}{\cos \left( {2A} \right)}} + {D_{6}\left( {R^{2} - 1} \right)} + {D_{7}R^{2}{\sin \left( {2A} \right)}} + {D_{8}R^{3}{\cos \left( {3A} \right)}} + {{D_{9}\left( {{3R^{3}} - {2R}} \right)}{\cos (A)}} + {{D_{10}\left( {{3R^{3}} - {2R}} \right)}{\sin (A)}} + {D_{11}R^{3}{\sin \left( {3A} \right)}} + {D_{12}R^{4}{\cos \left( {4A} \right)}} + {{D_{13}\left( {{4R^{4}} - {3R^{2}}} \right)}{\cos \left( {2A} \right)}} + {D_{14}\left( {{6R^{4}} - {6R^{2}} + 1} \right)} + {{D_{15}\left( {{4R^{4}} - {3R^{2}}} \right)}{\sin \left( {2A} \right)}} + {D_{16}R^{4}{\sin \left( {4A} \right)}} + {D_{17}R^{5}{\cos \left( {5A} \right)}} + {{D_{18}\left( {{5R^{5}} - {4R^{3}}} \right)}{\cos \left( {3A} \right)}} + {{D_{19}\left( {{10R^{5}} - {12R^{3}} + {3R}} \right)}{\cos (A)}} + {{D_{20}\left( {{10R^{5}} - {12R^{3}} + {3R}} \right)}{\sin (A)}} + {{D_{21}\left( {{5R^{5}} - {4R^{3}}} \right)}{\sin \left( {3A} \right)}} + {D_{22}R^{5}{\sin \left( {5A} \right)}} + {D_{23}R^{6}{\cos \left( {6A} \right)}} + {{D_{24}\left( {{6R^{6}} - {5R^{4}}} \right)}{\cos \left( {4A} \right)}} + {{D_{25}\left( {{15R^{6}} - {20R^{4}} + {6R^{2}}} \right)}{\cos \left( {2A} \right)}} + {D_{26}\left( {{20R^{6}} - {30R^{4}} + {12R^{2}} - 1} \right)} + {{D_{27}\left( {{15R^{6}} - {20R^{4}} + {6R^{2}}} \right)}{\sin \left( {2A} \right)}} + {{D_{28}\left( {{6R^{6}} - {5R^{4}}} \right)}{\sin \left( {4A} \right)}} + {D_{29}R^{6}{\sin \left( {6A} \right)}\quad \ldots}}}} & (d)\end{matrix}$

[0179] It should be noted that the plane-symmetry three-dimensionalsurface in the above equation is expressed as a surface which issymmetric with respect to the X-axis direction. In the above equation,D_(m) (m is an integer of 2 or higher) are coefficients.

[0180] A three-dimensional surface symmetric with respect to the X-axisdirection may be defined in correspondence to the above equation (c) asfollows: $\begin{matrix}\begin{matrix}{Z = {C_{2} +}} \\{{{{{{C_{3}Y} + C_{4}}}X}} +} \\{{{{{{C_{5}Y^{2}} + {C_{6}Y}}}X}} + {C_{7}X^{2}} +} \\{{{{{{{{{{C_{8}Y^{3}} + {C_{9}Y^{2}}}}X}} + {C_{10}{YX}^{2}} + C_{11}}}X^{3}}} +} \\{{{{{{{{{{C_{12}Y^{4}} + {C_{13}Y^{3}}}}X}} + {C_{14}Y^{2}X^{2}} + {C_{15}Y}}\quad }X^{3}}} + {C_{16}X^{4}} +} \\{{{{{{{{{{C_{17}Y^{5}} + {C_{18}Y^{4}}}}X}} + {C_{19}Y^{3}X^{2}} + {C_{20}Y^{2}}}}X^{3}}} + {C_{21}{YX}^{4}} + {C_{22}{X^{5}}} +} \\{{{{{{C_{23}Y^{6}} + {C_{24}Y^{5}}}}X}} + {C_{25}Y^{4}X^{2}} + {C_{26}Y^{3}{X^{3}}} + {C_{27}Y^{2}X^{4}} +} \\{{{{{{C_{28}Y\quad {X^{5}}} + {C_{29}X^{6}} + {C_{30}Y^{7}} + {C_{31}Y^{6}}}}X}} + {C_{32}Y^{5}X^{2}} +} \\{{{C_{33}Y^{4}{X^{3}}} + {C_{34}Y^{3}X^{4}} + {C_{35}Y^{2}{X^{5}}} + {C_{36}{YX}^{6}} + {C_{37}{X^{7}}}}}\end{matrix} & (e)\end{matrix}$

[0181] Incidentally, it is possible to construct an image displayapparatus for a single eye by preparing a combination of an ocularoptical system arranged as described above and an image display device.Alternatively, it is possible to construct an image display apparatusfor both eyes by preparing a pair of combinations of an ocular opticalsystem arranged as described above and an image display device for theleft and right eyes, and supporting them apart from each other by theinterpupillary distance, i.e. the distance between the two eyes. In thisway, it is possible to form a stationary or portable image displayapparatus which enables the observer to see with a single eye or botheyes.

[0182]FIG. 21 shows an image display apparatus designed for a single eye(in this case, the apparatus is designed for the left eye), and FIG. 22shows an image display apparatus designed for both eyes. In FIGS. 21 and22, reference numeral 31 denotes a display apparatus body unit. In thecase of FIG. 21, the display apparatus body unit 31 is supported by asupport member through the observer's head such that the displayapparatus body unit 31 is held in front of the observer's left eye. Inthe case of FIG. 22, the display apparatus body unit 31 is supported bya support member through the observer's head such that the displayapparatus body unit 31 is held in front of both the observer's eyes. Thesupport member has a pair of left and right front frames 32 each joinedat one end thereof to the display apparatus body unit 31. The left andright front frames 32 extend from the observer's temples to the upperportions of his/her ears, respectively. A pair of left and right rearframes 33 are joined to the other ends of the left and right frontframes 32, respectively, and extend over the left and right sideportions of the observer's head. In the case of FIG. 22, the supportmember further has a top frame 34 joined at both ends thereof to theother ends of the left and right rear frames 33, respectively, such thatthe top frame 34 supports the top of the observer's head.

[0183] A rear plate 35 is joined to one front frame 32 near the joint tothe rear frame 33. The rear plate 35 is formed from an elastic member,e.g. a metal leaf spring. In the case of FIG. 22, a rear cover 36, whichconstitutes a part of the support member, is joined to the rear plate 35such that the rear cover 36 can support the apparatus at a positionbehind the observer's ear in a region extending from the back part ofthe head to the base of the neck. A speaker 39 is mounted inside therear plate 35 or the rear cover 36 at a position corresponding to theobserver's ear.

[0184] A cable 41 for transmitting external image and sound signals isled out from the display apparatus body unit 31. In the case of FIG. 22,the cable 41 extends through the top frame 34, the rear frames 33, thefront frames 32 and the rear plate 35 and projects to the outside fromthe rear end of the rear cover 36. In the case of FIG. 21, the cable 41projects from the rear end of the rear plate 35. The cable 41 isconnected to a video reproducing unit 40. It should be noted thatreference numeral 40a denotes a switch and volume control part of thevideo reproducing unit 40.

[0185] The cable 41 may have a-jack and plug arrangement attached to thedistal end thereof so that the cable 41 can be detachably connected toan existing video deck. The cable 41 may also be connected to a TVsignal receiving tuner so as to enable the user to enjoy watching TV.Alternatively, the cable 41 may be connected to a computer to receivecomputer graphic images or message images or the like from the computer.To eliminate the bothersome cord, the image display apparatus may bearranged to receive external radio signals through an antenna connectedthereto.

[0186] Further, the ocular optical system of the image display apparatusaccording to the present invention can be used as an image-formingoptical system. For example, as shown in FIG. 23, the ocular opticalsystem may be used in a finder optical system F_(i) of a compact cameraC_(a) in which a photographic optical system O_(b) and the finderoptical system F_(i) are provided separately in parallel to each other.FIG. 24 shows the arrangement of an optical system in a case where theocular optical system according to the present invention is used as suchan image-forming optical system. As illustrated, the ocular opticalsystem DS according to the present invention is disposed behind a frontlens group GF and an aperture diaphragm D, thereby constituting anobjective optical system L_(t). An image that is formed by the objectiveoptical system L_(t) is erected by a Porro prism P, in which there arefour reflections, provided at the observer side of the objective opticalsystem L_(t), thereby enabling an erect image to be observed through anocular lens O_(c).

[0187] Although the prism optical element, image observation apparatusand image display apparatus according to the present invention have beendescribed above by way of some, examples, it should be noted that thepresent invention is not necessarily limited to these examples, and thatvarious modifications may be imparted thereto without departing from thescope of the present invention.

[0188] As will be clear from the foregoing description, the presentinvention makes it possible to provide an image display apparatus usableas an image observation apparatus which has an extremely thin andcompact ocular optical system and yet suffers from minimal unwantedlight and provides an observation image that is clear even at a wideobservation field angle.

1. A prism optical element comprising a plurality of surfaces facingeach other across a medium having a refractive index (n) larger than 1(n>1), wherein said plurality of surfaces include a first surface havingboth a transmitting action through which light rays enter said prismoptical element or exit therefrom and a reflecting action by which lightrays are internally reflected in said prism optical element; a secondsurface disposed to face said first surface across said medium andhaving a reflecting action by which light rays are internally reflectedin said prism optical element; a third surface disposed substantiallyclose to said second surface to face said first surface across saidmedium and having a reflecting action by which light rays are internallyreflected in said prism optical element; and a fourth surface havingsuch a transmitting action that when said first surface has an actionthrough which light rays enter said prism optical element, said fourthsurface has an action through which light rays exit from said prismoptical element, whereas, when said first surface has an action throughwhich light rays exit from said prism optical element, said fourthsurface has an action through which light rays enter said prism opticalelement, and wherein the following condition is satisfied: sin⁻¹(1/n_(d))≦θ_(r3)≦60°  (1) where n_(d) is a refractive index for the spectrald-line of said medium, and θ_(r3) is an angle of internal reflection ofan arbitrary light ray at said third surface.
 2. A prism optical elementaccording to claim 1, which satisfies the following condition: sin⁻¹(1/n_(d))≦θ_(r3)≦50°  (2)
 3. A prism optical element according to claim 1,wherein reflection at said first surface is total reflection.
 4. A prismoptical element according to claim 1, wherein the refractive index (n)of said 0 medium is larger than 1.3 (n>1.3).
 5. A prism optical elementaccording to claim 1, wherein at least one of surfaces constituting saidprism optical element is a plane surface.
 6. An observation opticalsystem comprising the prism optical element according to claim 1, saidprism optical element being disposed in an observation optical systemunit.
 7. An observation optical system according to claim 6, whereinsaid prism optical element is disposed in an objective lens.
 8. A camerafinder optical system comprising the observation optical system of claim6, wherein said prism optical element is disposed in image-erectingmeans disposed behind an objective lens to erect an object image formedby said objective lens.
 9. A camera finder optical system according toclaim 8, wherein said prism optical element has an ocular lens action inaddition to an image erecting action.
 10. A head-mounted image displayapparatus comprising: the prism optical element according to claim 1;image forming means disposed to face said fourth surface of said prismoptical element; and a retaining member that retains both said prismoptical element and said image forming means on an observer's facewherein a bundle of light rays emitted from said image forming meansenters said prism optical element through said fourth surface and passessequentially along an optical path in said prism optical element suchthat the light rays are reflected successively by said third surface,said first surface and said second surface and exit from said prismoptical element through said first surface.
 11. An image observationapparatus comprising image forming means and an ocular optical systemhaving an action by which an image formed by said image forming means isled to an eyeball of an observer, wherein said ocular optical systemincludes a prism member having at least three surfaces, wherein a spacebetween said at least three surfaces is filled with a single mediumhaving a refractive index (n) larger than 1 (n>1), said prism memberhaving an action by which light rays emitted from said image formingmeans are internally reflected at least three times, wherein at leasttwo of the at least three internal reflections are total reflections,and wherein at least one of the at least two total reflections isperformed by a surface disposed on a side of said single medium that iscloser to said observer, said surface being curved so as to correctaberrations produced by the internal reflections in said prism member,and wherein at least two of the at least three surfaces of said prismmember are disposed to face each other such that an external scene canbe observed through said at least two surfaces, and that a distortionproduced when the external scene is observed through said single mediumis minimized. 12-32. (canceled)
 33. An image observation apparatusaccording to claim 10, further comprising positioning means forpositioning said image forming means and said ocular optical system withrespect to an observer's head.
 34. An image observation apparatusaccording to claim 10, further comprising support means for supportingat least a pair of said image observation apparatuses at a predeterminedspacing.
 35. A prism optical element or prism member according to claim1, wherein said second surface and said third surface act as differentsurfaces in terms of optical action but are formed structurally from asingle surface.
 36. A prism optical element or prism member according toclaim 35, wherein said single surface constituting said second and thirdsurfaces is arranged such that a region of said surface closer to saidfourth surface acts as said third surface, and a region of said surfaceremote from said fourth surface acts as said second surface.
 37. A prismoptical element or prism member according to claim 36, wherein saidsingle surface constituting said second and third surfaces is arrangedsuch that a central region of said surface acts as both said second andthird surfaces.
 38. An image display apparatus comprising an imagedisplay device and an ocular optical system for leading an image formedby said image display device to an eyeball of an observer such that saidimage can be observed as a virtual image, wherein said ocular opticalsystem includes: a decentered prism in which a space formed by at leasttwo surfaces is filled with a medium having a refractive index largerthan 1, said at least two surfaces including a first surface positionedimmediately in front of the observer's eyeball, and a second surfacewhich is a reflecting surface facing said first surface, at least one ofsaid at least two surfaces being a curved surface decentered or tiltedwith respect to an observer's visual axis, and aberration correctingmeans disposed outside said second surface to correct aberrations due todecentration produced by said first and second surfaces with respect tolight from an external scene.
 39. An image display apparatus accordingto claim 38, wherein said aberration correcting means comprises aFresnel lens.
 40. An image display apparatus according to claim 39,wherein a center of an annular zone of said Fresnel lens lies in a planecontaining an optical path of an axial principal ray from said imagedisplay device, and said Fresnel lens is decentered perpendicularly tothe observer's visual axis in the plane containing the optical path ofthe axial principal ray.
 41. An image display apparatus according toclaim 39, wherein a center of an annular zone of said Fresnel lens liesin a plane containing an optical path of an axial principal ray fromsaid image display device, and said Fresnel lens is tilted with respectto the observer's visual axis so as to extend along a surfaceconfiguration of said second surface.
 42. An image display apparatusaccording to claim 38, wherein said aberration correcting meanscomprises a diffractive optical element.
 43. An image display apparatusaccording to claim 38, wherein said aberration correcting meanscomprises a holographic optical element.
 44. An image display apparatuscomprising an image display device and an ocular optical system forleading an image formed by said image display device to an eyeball of anobserver such that said image can be observed as a virtual image,wherein said ocular optical system includes a decentered prism in whicha space formed by at least three surfaces is filled with a medium havinga refractive index larger than 1; said at least three surfacesincluding: a refracting and internally reflecting surface positionedimmediately in front of said observer's eyeball; an outside world-sideinternally reflecting surface disposed on an outside world side of saidocular optical system to face said refracting and internally reflectingsurface; and a refracting surface through which a bundle of light raysemitted from said image display device enters said decentered prism,wherein at least one of said at least three surfaces is decentered ortilted with respect to an observer's visual axis, and said at leastthree surfaces are arranged to perform at least three internalreflections, said ocular optical system further includes a secondoptical element that cancels a power produced by said refracting andinternally reflecting surface, which is positioned immediately in frontof said observer's eyeball, and said outside world-side internallyreflecting surface with respect to external light when an external sceneis observed through said two surfaces, said second optical element beingdisposed on an outside world side of said outside world-side internallyreflecting surface.
 45. An image display apparatus according to claim44, wherein said ocular optical system comprises a decentered prism inwhich a space formed by four surfaces is filled with a medium having arefractive index larger than 1, said four surfaces including a firstsurface positioned on an observer's eyeball side of said ocular opticalsystem and serving as both refracting and reflecting surfaces, a secondsurface which is a reflecting surface disposed to face said firstsurface; a third surface which is a reflecting surface disposed to facesaid first surface at a position adjacent to said second surface; and afourth surface which is a refracting surface closest to said imagedisplay device, wherein at least one of said four surfaces is decenteredor tilted with respect to the observer's visual axis.
 46. An imagedisplay apparatus according to claim 45, wherein at least one secondoptical element is disposed on an outside world side of said second orthird surface so that an external scene can be observed through saidfirst surface, said second surface and said second optical element orthrough said first surface, said third surface and said second opticalelement.
 47. An image display apparatus according to claim 46, whereinsaid second optical element simultaneously cancels a composite power ofsaid first and second surfaces and a composite power of said first andthird surfaces with respect to light from the external scene.
 48. Animage display apparatus according to claims 38 or 44, further comprisingpositioning means for positioning said image display device and saidocular optical system with respect to an observer's head.
 49. An imagedisplay apparatus according to claims 38 or 44, further comprisingsupport means for supporting said image display device and said ocularoptical system with respect to an observer's head such that saidapparatus can be mounted on the observer's head.
 50. An image displayapparatus according to claims 38 or 44, further comprising support meansfor supporting at least a pair of said image display apparatuses at apredetermined spacing.
 51. An image display apparatus according toclaims 38 or 44, wherein said ocular optical system is used as animage-forming optical system.
 52. An optical system comprising: adecentered prism having at least two surfaces, wherein a space formed bysaid at least two surfaces is filled with a medium having a refractiveindex larger than 1; and aberration correcting means provided at aposition separate from said decentered prism; wherein at least one ofsaid at least two surfaces is a curved surface decentered or tilted withrespect to a predetermined axis, and wherein said aberration correctingmeans is provided so that light that has passed through said aberrationcorrecting means passes through said decentered prism.
 53. An opticalsystem comprising: a decentered prism having at least three surfaces,wherein a space formed by said at least three surfaces is filled with amedium having a refractive index larger than 1; and a second opticalelement provided at a position separate from said decentered prism;wherein at least one of said at least three surfaces is a curved surfacedecentered or tilted with respect to a predetermined axis; said at leastthree surfaces including a first surface and a second surface; saidsecond surface being positioned closest to said second optical element;said first surface being positioned to face said second optical elementacross said second surface; and wherein said second optical element isprovided so that light that has passed through said second opticalelement passes through said decentered prism, and said second opticalelement cancels a power produced by said second surface.